Revisit statefulness in OTLP MetricExporter

[codeboten]

As per the discussion here: #835 (comment)

The OTLP MetricExporter will need to be revisited as the metrics SDK, protos and spec evolve.

[jmacd]

This "stateless" concept is discussed in open-telemetry/opentelemetry-specification#1198. The concept found in this code predates the current OTel-Go design, which changed after I found design errors in the Prometheus export path. The SDK spec PR 1198 talks about two behaviors that cause statefulness to arise: (1) conversion from delta to cumulative for the counter instruments, (2) memory simply for the same of memory. Both of these are needed for Prometheus, which expects you to continue exporting the last sample of all timeseries that were ever exported from the process.