Add metric semantic conventions for timed operations

[justinfoote]

trafficstars

This OTEP attempts to address https://github.com/open-telemetry/opentelemetry-specification/issues/654.

I'd like us to come to agreement that we ought to have semantic conventions describing timing metrics. These semantic conventions will describe what instrument(s) are created, how they are named, and what labels are applied to them. They will not describe how or where the instruments are created and incremented.

[justinfoote]

@open-telemetry/specs-metrics-approvers would y'all have a look at this? I'd like us to come to an agreement sooner rather than later that we want some semantic conventions about naming and labeling of metrics, and that we're not intending to specify API or SDK changes with them.

[fatsheep9146]

Any update on this? @jmacd @MrAlias @justinfoote

[tedsuo]

@justinfoote we are marking OTEPs which are no longer being updated as stale. We will close this PR in one week. If you wish to resurrect this PR, that is fine.