Support gRPC health check service in VTGate

[shichao-an]

Feature Description

A written overview of the feature.

Currently the VTGate gRPC server doesn't register the health check service. Here's the output listing services of a VTGate server using grpcurl:

$ ./grpcurl -plaintext  localhost:15991 list
grpc.reflection.v1alpha.ServerReflection
vtgateservice.Vitess

The health check service can be registered like this: https://github.com/vitessio/vitess/blob/main/go/vt/vtadmin/grpcserver/server.go#L138

healthServer := health.NewServer()
healthpb.RegisterHealthServer(gserv, healthServer)

If the gRPC server has health check service registers the grpcurl ls output is like this (using temporal as an example)

grpc.health.v1.Health
grpc.reflection.v1alpha.ServerReflection
temporal.api.workflowservice.v1.WorkflowService
temporal.server.api.adminservice.v1.AdminService

https://github.com/vitessio/vitess/blob/main/go/vt/vtadmin/grpcserver/server.go#L138

Use Case(s)

In our ecosystem we use grpc.health.v1.Health endpoint to check the health of a gRPC server. It optionally takes a service name, such as vtgateservice.Vitess to query for health status.