Apply grpc service config from consul

[onyn]

We have DiscoveryClientNameResolver which allows to specify grpc client target like discovery:///service-name. Actual ip:port data is fetched from consul. Port also taken from gRPC_port metadata.

I suggest adding gRPC_service_config metadata key, similar to gRPC_port. Under this key grpc servers may publish service config.

This feature already present in standard DNS name resolver (details).

I already implement this in my custom name resolver and it work just fine.

On server side I specify my desired service config:

Server's application.yml

```yaml spring: cloud: consul: discovery: metadata: gRPC_service_config: | { "loadBalancingConfig": [ {"round_robin": {}} ], "methodConfig": [ { "name": [{}], "retryPolicy": { "maxAttempts": 5, "initialBackoff": "0.05s", "maxBackoff": "1s", "backoffMultiplier": 2, "retryableStatusCodes": [ "UNAVAILABLE", "ABORTED", "DATA_LOSS", "INTERNAL", "DEADLINE_EXCEEDED" ] }, "timeout": "5s" } ] } ```

On client side my name resolver parse it and pass to listener:

Code sample

```java var result = ResolutionResult.newBuilder().setAddresses(list); if (!serviceConfig.isEmpty()) { try { @SuppressWarnings("unchecked") Map parsed = gson.fromJson(serviceConfig, Map.class); result.setServiceConfig(serviceConfigParser.parseServiceConfig(parsed)); } catch (JsonSyntaxException e) { result.setServiceConfig( ConfigOrError.fromError( Status.UNKNOWN .withDescription("Failed to parse gRPC_service_config") .withCause(e) ) ); } } savedListener.onResult(result.build()); ```

[ST-DDT]

I already implement this in my custom name resolver and it work just fine.

Would you like to create a PR for that?