dashboard: add two new endpoints to dashboard for Nomad examples

[shoenig]

The Nomad team is rolling out configuration of Envoy expose paths
for its Consul Connect integration. In Nomad's context, configuring
expose paths allows for "punching a hole" through the network namespace
that Nomad automatically creates for Connect enabled tasks. To demonstrate
this capability, this CR updates the dashboard-service to expose 2
new endpoints. Nomad makes extensive use of these two services throughout
its documentation as a typical example of a Connect setup.

• /health/api: provides a real health-check against the connectivity
  with the back-end API service. Consecutive connection failures
  are counted, and if non-zero, the endpoint returns a 503. If
  the most recent connection attempt was success, returns 200.

• /metrics: simply exposes Go's built-in expvar Handler, which
  produces a JSON response full of Go runtime memory metrics and
  other such data.

The existing endpoints are left as-is.

Example usage:

launch counting service

$ PORT=8888 go run main.go

launch dashboard service

$ PORT=9999 COUNTING_SERVICE_URL='http://localhost:8888' go run main.go

show /metrics endpoint

$ curl  -s localhost:9999/metrics | jq . | head -n 10
{
  "cmdline": [
    "/tmp/go-build318152207/b001/exe/main"
  ],
  "memstats": {
    "Alloc": 1193440,
    "TotalAlloc": 1193440,
    "Sys": 71387144,
    "Lookups": 0,
    "Mallocs": 2822,
...

show /health/api endpoint with counting service up & running

$ curl -w '\ncode: %{response_code}\n' -s localhost:9999/health/api
ok
code: 200

show /health/api endpoint with counting service shutdown

$ curl -w '\ncode: %{response_code}\n' -s localhost:9999/health/api
failures: 3
code: 503