nomad-sd: Expose service check status as metrics

### Proposal
Expose service's health check status via metrics.
Ideally, it would be nice to have a gauge for healthy vs unhealthy allocations of a specific service.

### Use-cases
Allow external tools to detect cases in which service went unhealthy.

Start by locating the nomad-sd service-check handling and the existing metrics entry points in the Nomad codebase. Define how healthy and unhealthy allocations for a specific service should be represented, then verify that the resulting metrics expose the service health changes described in the proposal.

Written by the indexing model from the issue text.