grpc: use default resolver scheme for grpc dialing

[shoenig]

Currently checks of type gRPC will emit log messages such as,

2020/02/12 13:48:22 [INFO] parsed scheme: ""
2020/02/12 13:48:22 [INFO] scheme "" not registered, fallback to default scheme

Without adding full support for using custom gRPC schemes (perhaps that's
right long-term option) we can just supply the default scheme as provided
by the grpc library.

Fixes #7274
and transitively hashicorp/nomad#7415

Did some manual testing.

Without Proxy

Toy gRPC HC service

$ docker run --net=host --rm shoenig/grpc-healthcheck-example:v0.0.0

Consul config w/ check on service

# consul.hcl

log_level = "INFO"
data_dir = "/tmp/consul"
server = true
bootstrap_expect = 1
bind_addr = "127.0.0.1"
addresses {
  http = "0.0.0.0"
}

services = {
  name = "grpc-healthcheck-example"
  port = 3333

  check = {
    "id"           = "example-check"
    "name"         = "gRPC Check Example"
    "grpc"         = "127.0.0.1:3333/example"
    "grpc_use_tls" = false
    "interval"     = "7s"
  }
}

Run Consul

$  consul agent -config-file=consul.hcl

Verify check passes

$ curl -s localhost:8500/v1/agent/checks | jq .
{
  "example-check": {
    "Node": "x1",
    "CheckID": "example-check",
    "Name": "gRPC Check Example",
    "Status": "passing",
    "Notes": "",
    "Output": "gRPC check 127.0.0.1:3333/example: success",
    "ServiceID": "grpc-healthcheck-example",
    "ServiceName": "grpc-healthcheck-example",
    "ServiceTags": [],
    "Type": "grpc",
    "Definition": {},
    "CreateIndex": 0,
    "ModifyIndex": 0
  }
}

With Proxy (using Nomad)

Run Consul

$ consul agent -dev

Run Nomad

$ sudo nomad agent -dev-connect

Nomad job file w/ Connect enabled, exposed gRPC HC service

# example.nomad

job "grpc-check-example" {
  datacenters = ["dc1"]

  group "group1" {
    network {
      mode = "bridge"

      port "healthcheck" {
	to = -1
      }
    }



    service {
      name = "grpc-healthcheck-example"
      port = "healthcheck"

      connect {
	sidecar_service {}
      }

      check {
	name = "example-check"
	type = "grpc"
	port = "healthcheck"
	interval = "5s"
	timeout = "3s"
	expose = true
      }
    }

    task "example" {
      driver = "docker"

      env {
	GRPCHC_PORT = "${NOMAD_PORT_healthcheck}"
      }

      config {
	image = "shoenig/grpc-healthcheck-example:v0.0.0"
      }
    }
  }
}

Wait for job to become healthy

$ nomad job status grpc-check-example
ID            = grpc-check-example
Name          = grpc-check-example
Submit Date   = 2020-04-08T13:46:44-06:00
Type          = service
Priority      = 50
Datacenters   = dc1
Namespace     = default
Status        = running
Periodic      = false
Parameterized = false

Summary
Task Group  Queued  Starting  Running  Failed  Complete  Lost
group1      0       0         1        0       0         0

Latest Deployment
ID          = cf59d40b
Status      = successful
Description = Deployment completed successfully

Deployed
Task Group  Desired  Placed  Healthy  Unhealthy  Progress Deadline
group1      1        1       1        0          2020-04-08T13:56:59-06:00

Allocations
ID        Node ID   Task Group  Version  Desired  Status   Created  Modified
51972858  d3aa3a10  group1      0        run      running  31s ago  16s ago

Verify check is passing

$ curl -s localhost:8500/v1/agent/checks | jq '.[] | select(.Name=="example-check")'
{
  "Node": "x1",
  "CheckID": "_nomad-check-a20c0d5a5e2abd9d953c1731fbdcb5b185b7ef79",
  "Name": "example-check",
  "Status": "passing",
  "Notes": "",
  "Output": "gRPC check 192.168.1.252:30697/: success",
  "ServiceID": "_nomad-task-51972858-e1bd-0329-437a-81a942e34cd5-group-group1-grpc-healthcheck-example-healthcheck",
  "ServiceName": "grpc-healthcheck-example",
  "ServiceTags": [],
  "Type": "grpc",
  "Definition": {},
  "CreateIndex": 0,
  "ModifyIndex": 0
}

[hanshasselberg]

LGTM! Thank you for submitting a PR!