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

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

var result = ResolutionResult.newBuilder().setAddresses(list);
if (!serviceConfig.isEmpty()) {
    try {
        @SuppressWarnings("unchecked")
        Map<String, ?> 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?