[wip] Feat/go metrics

[coryschwartz]

fix: testground/testground#909

Similarly to the old prometheus metrics, I have placed the metrics namedpaced under runenv.M()
The metrics are output once every second.

go-metrics has a Log() method which accepts a logger, however the output is human-readable rather than machine-readable. This is clearly not what we want.

The simple way to write logs to metrics.out in json format is simply to open a Writer and write to it. In order to get zap logging to work, I added a simple Writer which writes to the zap log.

I think we don't need or want both. I would, I think, ditch the zap logging and keep the metrics.log output, @raulk what do you think?

[coryschwartz]

If you want to see what this plan does, you can use it with this plan:

testground/testground#917

The outputs collection includes a new file metrics.out which has json logging and there is also zap logging which lands in run.out. Either format works fine, in my opinion, although this will make a difference on how testground/testground#910 is implemented.

[coryschwartz]

If the output we want is zap logging in the metrics.out file, we can add another logger

[raulk]

See #3 for successor.

[raulk]

BTW -- this was a good start. I used the API that you laid out, and built on it to add all the backing InfluxDB interactions.