Support for synchronous guages

[ewoolsey]

Feature Request

Motivation

This feature obviously would fit in well here since it already supports the other synchronous metrics. Synchronous gauges were added in this PR and are listed as experimental. It is included in the open-telemetry spec though so I imagine it's unlikely to change much.

Proposal

Api should be consistent with existing metrics and look like this:

info!(gauge.foo = 1.1);

We could potentially feature gate this as experimental similar to open-telemetry. I'm not sure how necessary that is though.

[academiaresf]

I have a PR about but i need some help. I don't know the low level details to take about the Gauge metric on the test side.