Extract a `metrics_api` from the Job Metrics API processor in standard-assets

[godber]

In standard assets we added the following job_metric_api:

https://github.com/terascope/standard-assets/blob/master/asset/src/job_metric_api/api.ts

We should extract out the API into Teraslice itself so the metrics can be used elsewhere

See registerApis here:

https://github.com/terascope/teraslice/blob/master/packages/job-components/src/register-apis.ts#L113

... and the APIs that get registered could be good examples.

Note that when this issue is done we need to do the following:

• updated job_metrics_api to use the built in API
• add exporter to the Teraslice master
• add exporter to the Teraslice execution controller
• add exporter to the Teraslice worker

[godber]

Play with prom-client a bit:

https://github.com/siimon/prom-client

[busma13]

We decided that the proper place for this to live is in the terafoundation foundation APIS. https://github.com/terascope/teraslice/blob/master/packages/terafoundation/src/api/index.ts

[busma13]

The registerApis function within terafoundation contains the foundationApis object. It will now have a promMetrics key that contains all the functions related to the prom-metrics-api.

There was a lot of discussion about when the API and the associated exporter server should be created. I've experimented with creating the API at the terafoundation level vs the teraslice master, execution_controller, and worker level.

• Doing it in terafoundation seems out of order as the API setup requires adding logic that requires knowledge of the execution that will be run. If we want to use this API in other internal projects that use terafoundation we would need to add extra logic into terafoundation for that as well.
• Creating the API at the teraslice level requires adding some logic to the constructors of each of the appropriate classes. All the necessary data is readily available in the constructors.