Support Prometheus Gauge Histogram

[alolita]

See https://github.com/jsuereth/opentelemetry-specification/blob/wip-prometheus-datamodel-spec/specification/metrics/datamodel.md#prometheus-compatibility

for SDKs and Collector

[jsuereth]

Duplicate of #51

[jsuereth]

Also related open-telemetry/opentelemetry-proto#274

[hdost]

Should we just close this since there's already an open issue

[VineethReddy02]

@jsuereth @alolita Is gauge histogram natively supported in Prometheus? It looks like Prometheus doesn't list it in the docs https://prometheus.io/docs/concepts/metric_types/

IMO the below metrics types are part of OpenMetrics which aren't used in Prometheus yet :/

1. Gaugehistogram
2. Info
3. Stateset

WDYT?