Clarify treatment of unspecified values in Metrics export pipeline

[jmacd]

This was discussed in #265, see this comment: #265 (comment)

The current otel-proto declares that unspecified values will be exported as empty strings. The question for the SDK is whether the exporters should be able to distinguish unspecified from empty strings. It could be accomplished by using the core.Value INVALID type, but the fix is not trivial because label encoders will need a way to represent unspecified.

[jmacd]

It has been discussed in the 2/27/20 Metrics SIG and I believe we have converged on treating the empty value as equivalent to the unspecified value.

As motivational evidence, note this line in the Prometheus documentation:

A label with an empty label value is considered equivalent to a label that does not exist.

This means that we should change the SDK's Meter.Labels(...) to drop empty-string values from the label set.

[jmacd]

The clarification is here: #345

We'll keep the current behavior, in other words, closing this.