feat(prometheus): add ContextLabels to ClientMetrics

[ArtARTs36]

This PR implements #783 and adds methods for adding labels from context.

Changes

• Add WithClientContextLabels option to ClientMetrics, similar WithContextLabels
• Changed internal clientMetricsConfig to retain the ability to pass options WithContextLabels, WithClientHandlingTimeHistogram, WithClientStreamRecvHistogram, WithClientStreamSendHistogram in any order. Histograms that are created in options must receive label names that are also created in options.

Verification

1. Init ClientMetrics with label names

clMetrics := grpcprom.NewClientMetrics(grpcprom.WithClientContextLabels("user_id"))

2. Create gRPC client with interceptor and WithLabelsFromContext

labelsFromContext := func(ctx context.Context) prometheus.Labels {
	return prometheus.Labels{"user_id": "admin"}
}
client, err := grpc.NewClient(targetGRPCAddr, grpc.WithChainUnaryInterceptor(
	clMetrics.UnaryClientInterceptor(
		grpcprom.WithLabelsFromContext(labelsFromContext), 
	)))

3. Send request and given new label in counter

grpc_client_handled_total{grpc_code="OK",grpc_method="Ping",grpc_service="testing.testpb.v1.TestService",grpc_type="unary",user_id="admin"} 12

[johanbrandhorst]

This looks great, thank you! I'm not sure about the test failure, that doesn't look like it has anything to do with this PR, does it fail locally for you too?

[ArtARTs36]

This passed for me locally. I suggest restarting this in GitHub Actions.

[johanbrandhorst]

Good enough for me, it looks like a flaky test

[johanbrandhorst]

Thanks for your contribution!