Copy gRPC trailers metadata for observation context consumption (#4612)

According to gRPC Javadoc, the metadata is not thread-safe.
Similar to the approach taken for handling headers metadata, this
commit ensures that the trailers metadata is also copied for later
consumption by `GrpcServerObservationContext`.

Signed-off-by: Tadaya Tsuyukubo <[email protected]>

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/grpc/ObservationGrpcServerCall.java

@@ -61,9 +61,12 @@ public void close(Status status, Metadata trailers) {
         if (status.getCause() != null) {
             this.observation.error(status.getCause());
         }
+        // Per javadoc, trailers are not thread-safe. Make a copy.
+        Metadata trailersToKeep = new Metadata();
+        trailersToKeep.merge(trailers);
         GrpcServerObservationContext context = (GrpcServerObservationContext) this.observation.getContext();
         context.setStatusCode(status.getCode());
-        context.setTrailers(trailers);
+        context.setTrailers(trailersToKeep);
         super.close(status, trailers);
     }