chore: export internal DirectAccess gRPC metrics to the new bigtable_client monitored resource (#2547)

This will export a relevant subset of the metrics described in https://togithub.com/grpc/proposal/blob/master/A78-grpc-metrics-wrr-pf-xds.md as internal metrics to help rollout of DirectAccess

Change-Id: Ifddadb84d091f0f904f8f0f6e8da52de552e9757

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Fixes #<issue_number_goes_here> ☕️

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Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/metrics/BuiltinMetricsConstants.java

@@ -64,6 +64,7 @@ public class BuiltinMetricsConstants {
   static final String CLIENT_BLOCKING_LATENCIES_NAME = "throttling_latencies";
   static final String PER_CONNECTION_ERROR_COUNT_NAME = "per_connection_error_count";
 
+  // Start allow list of metrics that will be exported as internal
   public static final Map<String, Set<String>> GRPC_METRICS =
       ImmutableMap.<String, Set<String>>builder()
           .put(
@@ -95,6 +96,7 @@ public class BuiltinMetricsConstants {
       ImmutableSet.of(PER_CONNECTION_ERROR_COUNT_NAME).stream()
           .map(m -> METER_NAME + m)
           .collect(ImmutableSet.toImmutableSet());
+  // End allow list of metrics that will be exported
 
   // Buckets under 100,000 are identical to buckets for server side metrics handler_latencies.
   // Extending client side bucket to up to 3,200,000.