Implement metrics API (Addresses #12)

[mpardalos]

This implements the metrics part of the OpenTelemetry API (https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md). An explanation of the API is available in opentelemetry/src/OpenTelemetry/Metrics.hs

Caveats:

• Uses text messages
• No tags --- not directly supported by chrome-tracing, but we could maybe find some kind of hack?

[mpardalos]

I'm just realizing that I now need to switch over to the binary eventlog to add metrics to the tests, since I need to add MetricEv here: https://github.com/ethercrow/opentelemetry-haskell/blob/master/opentelemetry-extra/unit-tests/LogEventSerializer.hs#L15-L25