Reimplement teraslice-exporter using new foundation promMetrics api

[godber]

When this (https://github.com/terascope/teraslice/pull/3594/files) is merged, we should move the external teraslice-exporter project (found here: https://github.com/terascope/teraslice-exporter) into the core of the Teraslice master and adapt it to use the promMetrics API. It should expose the same set of metrics but no longer gather it's information by querying the Teraslice api.

[godber]

@busma13 can close this when he's validated internally.

[godber]

We want to try the following tests in a test environment:

• Teraslice v1.5.0, built-in promMetrics disabled, run job, see master resource usage
• Teraslice v1.5.0, built-in promMetrics enabled, run job, see master,worker,exc resource usage
• Teraslice v1.5.0, built-in promMetrics enabled, run job, scrape with prometheus, see master,worker,exc resource usage

Other things may be required in the job, but in order to target the job with prometheus, at the moment, we need to label the job manually like this on the job definition:

    ...
    "labels": {
        "scrape-target": "true",
        "scrape-prometheus-instance": "my-prom"
    },
   ...