agents,lib,src,test: add traceSampleRate support

Add end-to-end traceSampleRate handling across config, runtime
propagation, tracing decisions, and regression tests.

Why:
- Enable configurable probabilistic trace sampling with predictable
  behavior.
- Ensure consistent semantics across all config entry points.
- Prevent invalid updates from corrupting current sampling behavior.
- Keep transaction consistency by deciding sampling at the root span
  only.

What changed:
- Added traceSampleRate parsing and normalization in JS config paths
  with explicit default fallback and finite/range validation in [0, 1].
- Added native config sanitization for traceSampleRate to reject invalid
  values before merge, preserving previous valid configuration.
- Ensured runtime sampling state is synchronized from effective current
  config after updates to avoid stale shared-memory sample rates.
- Added gRPC reconfigure support for traceSampleRate in proto and agent
  mapping, including generated protobuf updates.
- Updated tracing logic so root spans perform the sampling decision and
  child spans inherit parent traceFlags.
- Extended tests for:
  - invalid value handling (including NaN/Infinity)
  - env/package bootstrap behavior
  - partial updates preserving existing traceSampleRate
  - gRPC invalid-update fallback behavior
  - sampling behavior at 0%, 50% (tolerance), and 100%
  - worker-thread sampling behavior
  - explicit parent/child trace consistency assertions

Author: santigimeno

agents/grpc/proto/reconfigure.proto

@@ -18,6 +18,7 @@ message ReconfigureBody {
   optional uint32 tracingModulesBlacklist = 11;
   optional bool contCpuProfile = 12;
   optional bool assetsEnabled = 13;
+  optional double traceSampleRate = 14;
 }
 
 message ReconfigureEvent {

agents/grpc/src/grpc_agent.cc

@@ -1758,6 +1758,10 @@ void GrpcAgent::reconfigure(const grpcagent::CommandRequest& request) {
       out["assetsEnabled"] = body.assetsenabled();
   }
 
+  if (body.has_tracesamplerate()) {
+      out["traceSampleRate"] = body.tracesamplerate();
+  }
+
   DebugJSON("Reconfigure out: \n%s\n", out);
 
   UpdateConfig(out.dump());

agents/grpc/src/proto/reconfigure.pb.cc

@@ -2,6 +2,7 @@
 
 const {
   JSONStringify,
+  MathRandom,
   SafeMap,
   Uint8Array,
 } = primordials;
@@ -278,10 +279,14 @@ class Tracer {
     }
 
     const internalId = newInternalSpanId();
-    // Follow parent, root always sampled.
-    let flags = api.TraceFlags.SAMPLED;
-    if (parentContext)
+    // Root spans make the sampling decision, child spans inherit parent flags.
+    let flags;
+    if (!parentContext) {
+      const sampled = MathRandom() < binding.trace_sample_rate[0];
+      flags = sampled ? api.TraceFlags.SAMPLED : api.TraceFlags.NONE;
+    } else {
       flags = parentContext.traceFlags;
+    }
     const spanContext = new SpanContext(internalId,
                                         spanId,
                                         flags,