Clarify Metric data type

[reyang]

Fixes #2073.

This change adds extra clarification, it doesn't change the spec requirement.

[cijothomas]

lgtm

[MrAlias]

This tries to clarify the types that implementations can use, but I think the original issue that this was opened to resolve is that the term Metric itself is not well defined. Here in the specification we are using the term to describe a group of MetricPoints, but above it also describes a "metric time series". This seems like a overloading of the term.

Additionally, above we refer to Aggregated Metrics above, but it seems to be referring to the same conceptual thing we a describing here.

I think adding these changes can help clarify implementation details for implementers, but we should also be consistent in our use of terms. I would recommend using Aggregated Metrics here if they are the same as above, or maybe instead just say "a batch of MetricPoint groups".

[reyang]

I think adding these changes can help clarify implementation details for implementers, but we should also be consistent in our use of terms. I would recommend using Aggregated Metrics here if they are the same as above, or maybe instead just say "a batch of MetricPoint groups".

I've updated the wording, PTAL.