Configure custom metadata on gRPC healthchecks

#### Feature Description

When configuring gRPC healthchecks, it might be useful to add metadata to the gRPC request, similarly to what we can do for HTTP healthchecks with HTTP headers.

Considering the example retrieved from your docs with added fields:

```
check = {
id = "mem-util"
name = "Service health status"
grpc = "127.0.0.1:12345/my_service"
grpc_use_tls = true
+ grpc_metadata = {
+ user-agent = "xxxx"
+ }
interval = "10s"
}
```

#### Use Case(s)

It can be used to:
* Set specific metadata keys to identify that the request is coming from Consul
* Set specific tracing information