Reword metrics spec to clarify what "client" means

[aabmass]

In both the metrics API and SDK specs, I see a mix of "the API", "the client", and "individual language client" which are all referring to a hypothetical implementation of the spec. I think "client" is a bit overloaded and it's unclear if this refers to an implementation or the developer using the implementation. In the tracing spec, I do also see "OpenTelemetry client authors".

I'm happy to take make a PR trying to refine the wording @reyang.

[reyang]

@aabmass I suggest that we align with "OpenTelemetry client authors".

[aabmass]

@reyang do you mean changing "the client" and "individual language client" to "OpenTelemetry client authors"? I personally like "the API"/"the SDK" the most. It is the most clear to me, highlights the different responsibilities of API vs SDK, and is the most widely used in the tracing spec.

[reyang]

@aabmass I've created a PR #1930 combining the ideas from the discussion here. Please help to review.