fix(trace-stats)!: add grpc_method to aggregation key (#2151)

# What does this PR do?

Removes \`grpc_method\` / \`rpc.method\` entirely from the OTLP trace metrics pipeline.

Previously \`grpc_method\` was a sidecar on \`GroupedStats\` — present in the OTLP metric payload as an explicit attribute, but not part of the aggregation key. This PR removes it from both:

- **\`FixedAggregationKey\`** — it is no longer an aggregation dimension.
- **\`OtlpExactGroup\`** — it is no longer carried in the per-cell sidecar.
- **\`build_attributes()\`** — \`rpc.method\` is no longer emitted on OTLP histogram data points.

## Rationale

For gRPC spans, \`span.resource\` already carries the full method path (e.g. \`/package.Service/Method\`). Emitting \`rpc.method\` as a separate OTLP attribute is redundant — \`span.resource\` (mapped to \`span.name\` on the data point) is the authoritative carrier of gRPC method identity and is already an aggregation dimension.

Adding \`rpc.method\` as a separate dimension would also increase cardinality for gRPC services without providing any information not already present on the metric.

Note: \`rpc.response.status_code\` (from \`grpc.status.code\`) is **kept** as a first-class aggregation dimension — it is not redundant with any existing field.

# Motivation

Correctness and cleanliness for the OTLP trace metrics feature landed in #2067.

# Risk

**Breaking changes (major semver):**

- \`FixedAggregationKey<T>\` loses its \`grpc_method: T\` field. Callers constructing the struct by name must remove the field.
- \`OtlpExactGroup\` loses its \`grpc_method\` field.
- \`build_attributes()\` loses the \`grpc_method\` parameter.

**Blast radius is limited** because:

1. The OTLP trace metrics feature (#2067) landed on \`main\` after the v36 release (June 19). No released version is affected — this only exists in unreleased code.
2. The OTLP metrics path is opt-in: the sidecar only activates it when \`set_otlp_metrics_endpoint\` is configured. No SDK enables this today.
3. Agent \`/v0.6/stats\` protobuf wire format is unchanged — \`ClientGroupedStats\` has no \`grpc_method\` field.

**Urgency: low.** No live production systems are affected. This must land before the next release that ships the OTLP metrics feature.

# How to test the change?

```
cargo nextest run -p libdd-trace-stats -p datadog-ipc -p libdd-data-pipeline
```

Co-authored-by: munir.abdinur <munir.abdinur@datadoghq.com>

Author: mabdinur

datadog-ipc/src/shm_stats.rs

@@ -60,7 +60,7 @@ use libdd_trace_stats::span_concentrator::{FixedAggregationKey, FlushableConcent
 
 use crate::platform::{FileBackedHandle, MappedMem, NamedShmHandle};
 
-const SHM_VERSION: u32 = 1;
+const SHM_VERSION: u32 = 2;
 
 /// Maximum peer-tag (key, value) pairs per aggregation slot.
 pub const MAX_PEER_TAGS: usize = 16;

libdd-data-pipeline/src/otlp/metrics.rs

@@ -58,7 +58,7 @@ pub fn map_stats_to_otlp_metrics(
                     continue;
                 };
                 data_points.push(json!({
-                    "attributes": build_attributes(group, &exact.grpc_method, is_error, resource_info, otel_trace_semantics_enabled),
+                    "attributes": build_attributes(group, is_error, resource_info, otel_trace_semantics_enabled),
                     "startTimeUnixNano": b.bucket.start.to_string(),
                     "timeUnixNano": end.to_string(),
                     "count": cell.count.to_string(),
@@ -120,7 +120,6 @@ fn sketch_bucket_counts(sketch: &DDSketch) -> Vec<String> {
 
 fn build_attributes(
     group: &pb::ClientGroupedStats,
-    grpc_method: &str,
     is_error: bool,
     resource_info: &OtlpResourceInfo,
     otel_trace_semantics_enabled: bool,
@@ -146,7 +145,6 @@ fn build_attributes(
     push("span.kind", &group.span_kind);
     push("http.request.method", &group.http_method);
     push("http.route", &group.http_endpoint);
-    push("rpc.method", grpc_method);
     push("rpc.response.status_code", &group.grpc_status_code);
     for tag in &group.peer_tags {
         if let Some((k, v)) = tag.split_once(':') {
@@ -347,22 +345,10 @@ mod tests {
             OtlpExactGroup {
                 ok: cell(ok_ns),
                 error: cell(err_ns),
-                grpc_method: String::new(),
             },
         )
     }
 
-    fn group_pair_with_grpc(
-        ok_ns: &[u64],
-        err_ns: &[u64],
-        grpc_method: &str,
-        customize: impl FnOnce(&mut pb::ClientGroupedStats),
-    ) -> (pb::ClientGroupedStats, OtlpExactGroup) {
-        let (g, mut e) = group_with_exact(ok_ns, err_ns, customize);
-        e.grpc_method = grpc_method.into();
-        (g, e)
-    }
-
     fn buckets(groups: Vec<(pb::ClientGroupedStats, OtlpExactGroup)>) -> Vec<OtlpStatsBucket> {
         let (stats, exact): (Vec<_>, Vec<_>) = groups.into_iter().unzip();
         vec![OtlpStatsBucket {
@@ -444,7 +430,7 @@ mod tests {
 
     #[test]
     fn data_point_attributes_and_otel_strip() {
-        let g_pair = group_pair_with_grpc(&[1_000_000_000], &[], "/pkg.Svc/Method", |g| {
+        let g_pair = group_with_exact(&[1_000_000_000], &[], |g| {
             g.http_status_code = 404;
             g.http_method = "POST".into();
             g.http_endpoint = "/users/:id".into();
@@ -467,7 +453,6 @@ mod tests {
         assert_eq!(str_at(a, "http.request.method"), Some("POST"));
         assert_eq!(str_at(a, "http.route"), Some("/users/:id"));
         assert!(a.iter().any(|kv| kv["key"] == "http.response.status_code"));
-        assert_eq!(str_at(a, "rpc.method"), Some("/pkg.Svc/Method"));
         assert_eq!(str_at(a, "datadog.operation.name"), Some("test.op"));
         assert_eq!(str_at(a, "datadog.span.type"), Some("web"));
         assert_eq!(str_at(a, "datadog.origin"), Some("synthetics"));

libdd-trace-stats/src/span_concentrator/aggregation.rs

@@ -24,7 +24,6 @@ const GRPC_STATUS_CODE_FIELD: &[&str] = &[
     "rpc.grpc.status.code",
     "grpc.status.code",
 ];
-const GRPC_METHOD_FIELD: &[&str] = &["grpc.method.name", "rpc.method"];
 
 /// Aggregation key fields shared across all concentrator implementations — everything
 /// **except** peer tags.
@@ -153,17 +152,6 @@ fn get_grpc_status_code<'a>(span: &'a impl StatSpan<'a>) -> Option<u8> {
     None
 }
 
-pub(super) fn get_grpc_method<'a>(span: &'a impl StatSpan<'a>) -> &'a str {
-    for key in GRPC_METHOD_FIELD {
-        if let Some(val) = span.get_meta(key) {
-            if !val.is_empty() {
-                return val;
-            }
-        }
-    }
-    ""
-}
-
 fn grpc_status_str_to_int_value(v: &str) -> Option<u8> {
     if let Ok(status) = v.parse() {
         return Some(status);
@@ -262,7 +250,6 @@ impl<'a> BorrowedAggregationKey<'a> {
         };
 
         let grpc_status_code = get_grpc_status_code(span);
-
         let service_source = span.get_meta(TAG_SVC_SRC).unwrap_or_default();
 
         Self {
@@ -344,9 +331,6 @@ pub(super) struct GroupedStats {
     error_duration: u64,
     error_min: u64,
     error_max: u64,
-    // gRPC method for OTLP export only; not part of the aggregation key so agent stats are
-    // unaffected.
-    pub(super) grpc_method: String,
 }
 
 impl GroupedStats {
@@ -390,14 +374,11 @@ pub struct OtlpExactCell {
 }
 
 /// Exact OK/ERROR cells for one aggregation group, in the same order as the `stats` vector
-/// of the accompanying [`pb::ClientStatsBucket`]. `grpc_method` is the group's gRPC method (DD
-/// schema `grpc.method.name`) carried out-of-band so it does not appear in the agent stats
-/// protobuf wire format.
+/// of the accompanying [`pb::ClientStatsBucket`].
 #[derive(Debug, Clone, Default)]
 pub struct OtlpExactGroup {
     pub ok: OtlpExactCell,
     pub error: OtlpExactCell,
-    pub grpc_method: String,
 }
 
 /// A bucket flushed for the OTLP trace-metrics path. `exact[i]` is the exact-scalar sidecar
@@ -433,14 +414,10 @@ impl StatsBucket {
         duration: i64,
         is_error: bool,
         is_top_level: bool,
-        grpc_method: &str,
     ) {
         self.data
             .entry_ref(&key)
-            .or_insert_with(|| GroupedStats {
-                grpc_method: grpc_method.to_owned(),
-                ..Default::default()
-            })
+            .or_default()
             .insert(duration, is_error, is_top_level);
     }
 
@@ -455,8 +432,7 @@ impl StatsBucket {
     pub(super) fn flush_with_otlp_exact(self, bucket_duration: u64) -> OtlpStatsBucket {
         let mut stats = Vec::with_capacity(self.data.len());
         let mut exact = Vec::with_capacity(self.data.len());
-        for (k, mut g) in self.data {
-            let grpc_method = std::mem::take(&mut g.grpc_method);
+        for (k, g) in self.data {
             exact.push(OtlpExactGroup {
                 ok: OtlpExactCell {
                     count: g.hits.saturating_sub(g.errors),
@@ -470,7 +446,6 @@ impl StatsBucket {
                     min_ns: g.error_min,
                     max_ns: g.error_max,
                 },
-                grpc_method,
             });
             stats.push(encode_grouped_stats(k, g));
         }
@@ -834,18 +809,6 @@ mod tests {
                 }
                 .into_key(),
             ),
-            // grpc.method.name is carried in GroupedStats (for OTLP), not in the aggregation key.
-            (
-                SpanBytes {
-                    meta: vec![("grpc.method.name".into(), "/pkg.Svc/Method".into())].into(),
-                    ..Default::default()
-                },
-                FixedAggregationKey {
-                    is_trace_root: true,
-                    ..Default::default()
-                }
-                .into_key(),
-            ),
             // Span with grpc status from meta as numeric string
             (
                 SpanBytes {

libdd-trace-stats/src/span_concentrator/mod.rs

@@ -9,7 +9,7 @@ use libdd_trace_protobuf::pb;
 use aggregation::StatsBucket;
 
 mod aggregation;
-use aggregation::{get_grpc_method, BorrowedAggregationKey};
+use aggregation::BorrowedAggregationKey;
 pub use aggregation::{FixedAggregationKey, OtlpExactCell, OtlpExactGroup, OtlpStatsBucket};
 
 pub mod stat_span;
@@ -184,7 +184,6 @@ impl SpanConcentrator {
                 span.duration(),
                 span.is_error(),
                 span.has_top_level(),
-                get_grpc_method(span),
             );
     }