feat: Set sampledToLocalTracing flag to true for all gRPC MethodDescriptors (#3874)

See b/416626151 for more information.

Problem: When the sampledToLocalTracing is set to false or not set, the
gRPC specific metrics have the `grpc.method` attribute defined as
`other` instead of the methodDescriptor's full name. This may be
confusing to customers who may expect to see the full gRPC method name.

High Level: This flag enables captures for specific method names to help
provide more detailed metrics. We do not expect any noticeable
performance impact from this change.

This does not impact the method name that is already properly captured
for Gax Metrics (e.g. Attempt Count, Operation Latency, etc).
Specifically, the use must be using grpc-java telemetry module and be
using
[GrpcOpenTelemetry](https://grpc.github.io/grpc-java/javadoc/io/grpc/opentelemetry/GrpcOpenTelemetry.html)
instead of
[OpenTelemetry](https://github.com/open-telemetry/opentelemetry-java/blob/main/api/all/src/main/java/io/opentelemetry/api/OpenTelemetry.java).

---------

Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: lqiu96

java-showcase/gapic-showcase/src/main/java/com/google/showcase/v1beta1/stub/GrpcEchoStub.java

@@ -119,6 +119,7 @@ public class GrpcEchoStub extends EchoStub {
                   ProtoUtils.marshaller(FailEchoWithDetailsRequest.getDefaultInstance()))
               .setResponseMarshaller(
                   ProtoUtils.marshaller(FailEchoWithDetailsResponse.getDefaultInstance()))
+              .setSampledToLocalTracing(true)
               .build();
 
   private static final MethodDescriptor<ExpandRequest, EchoResponse> expandMethodDescriptor =