feat: Add gRPC A66/A94 metrics.

This change adds the `grpc.client.attempt.started` metric in
the
[A66](https://github.com/grpc/proposal/blob/master/A66-otel-stats.md)
proposal and all the subchannel metrics in the
[A94](https://github.com/grpc/proposal/blob/master/A94-subchannel-otel-metrics.md)
proposal. Metric-specific attributes were also added.

The change also updates the metric exporter to use the same timestamp
for start and end time for GAUGE type data point. This is required by
the StackDriver or otherwise the export will fail.

Author: HailongWen

google-cloud-spanner/src/main/java/com/google/cloud/spanner/BuiltInMetricsConstant.java

@@ -70,6 +70,11 @@ public class BuiltInMetricsConstant {
 
   static final Collection<String> GRPC_METRICS_TO_ENABLE =
       ImmutableList.of(
+          "grpc.client.attempt.started",
+          "grpc.subchannel.open_connections",
+          "grpc.subchannel.disconnections",
+          "grpc.subchannel.connection_attempts_succeeded",
+          "grpc.subchannel.connection_attempts_failed",
           "grpc.lb.rls.default_target_picks",
           "grpc.lb.rls.target_picks",
           "grpc.xds_client.server_failure",
@@ -100,8 +105,6 @@ public class BuiltInMetricsConstant {
       AttributeKey.stringKey("directpath_used");
   public static final AttributeKey<String> REQUEST_ID_KEY =
       AttributeKey.stringKey(REQUEST_ID_HEADER_NAME);
-  public static final AttributeKey<String> GRPC_XDS_RESOURCE_TYPE_KEY =
-      AttributeKey.stringKey("grpc.xds.resource_type");
   public static Set<String> ALLOWED_EXEMPLARS_ATTRIBUTES =
       new HashSet<>(Arrays.asList(REQUEST_ID_HEADER_NAME));
 
@@ -124,8 +127,16 @@ public class BuiltInMetricsConstant {
           DIRECT_PATH_ENABLED_KEY,
           DIRECT_PATH_USED_KEY);
 
-  static final Set<String> GRPC_LB_RLS_ATTRIBUTES =
-      ImmutableSet.of("grpc.lb.rls.data_plane_target", "grpc.lb.pick_result");
+  static final Set<String> GRPC_ATTRIBUTES =
+      ImmutableSet.of(
+          "grpc.disconnect_error",
+          "grpc.method",
+          "grpc.target",
+          "grpc.lb.backend_service",
+          "grpc.lb.locality",
+          "grpc.lb.pick_result",
+          "grpc.lb.rls.data_plane_target",
+          "grpc.xds.resource_type");
 
   static List<Double> BUCKET_BOUNDARIES =
       ImmutableList.of(
@@ -140,7 +151,6 @@ public class BuiltInMetricsConstant {
       ImmutableList.of(
           "grpc.client.attempt.sent_total_compressed_message_size",
           "grpc.client.attempt.rcvd_total_compressed_message_size",
-          "grpc.client.attempt.started",
           "grpc.client.attempt.duration",
           "grpc.client.call.duration");
 
@@ -235,8 +245,7 @@ private static void defineGRPCView(ImmutableMap.Builder<InstrumentSelector, View
           BuiltInMetricsConstant.COMMON_ATTRIBUTES.stream()
               .map(AttributeKey::getKey)
               .collect(Collectors.toSet());
-      attributesFilter.addAll(BuiltInMetricsConstant.GRPC_LB_RLS_ATTRIBUTES);
-      attributesFilter.add(BuiltInMetricsConstant.GRPC_XDS_RESOURCE_TYPE_KEY.getKey());
+      attributesFilter.addAll(BuiltInMetricsConstant.GRPC_ATTRIBUTES);
 
       View view =
           View.builder()

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerCloudMonitoringExporterUtils.java

@@ -116,9 +116,10 @@ private static TimeSeries convertPointToSpannerTimeSeries(
       PointData pointData,
       MonitoredResource.Builder monitoredResourceBuilder,
       String projectId) {
+    MetricKind metricKind = convertMetricKind(metricData);
     TimeSeries.Builder builder =
         TimeSeries.newBuilder()
-            .setMetricKind(convertMetricKind(metricData))
+            .setMetricKind(metricKind)
             .setValueType(convertValueType(metricData.getType()));
     Metric.Builder metricBuilder = Metric.newBuilder().setType(metricData.getName());
 
@@ -143,7 +144,11 @@ private static TimeSeries convertPointToSpannerTimeSeries(
 
     TimeInterval timeInterval =
         TimeInterval.newBuilder()
-            .setStartTime(Timestamps.fromNanos(pointData.getStartEpochNanos()))
+            .setStartTime(
+                // For gauge metrics, the start and end time should be the same.
+                metricKind == MetricKind.GAUGE
+                    ? Timestamps.fromNanos(pointData.getEpochNanos())
+                    : Timestamps.fromNanos(pointData.getStartEpochNanos()))
             .setEndTime(Timestamps.fromNanos(pointData.getEpochNanos()))
             .build();