Add Prometheus Exporter - Utils Implementation

[CunjunWang]

This PR implements exporter utils helper functions in the exporter/prometheus/src folder for the Prometheus Exporter project. The C++ file prometheus_exporter_utils.cc implements all the helper functions that will be called when we translate a OpenTelemetry (OTLP) metric to a Prometheus metric.

Note to reviewers:

1. Please keep in mind that there are other PRs related to this one so codecov tests may not fully pass.
2. There are two TODOs that will be implemented and modified later:
   1. Translating from Sketch Aggregator to Prometheus Sketch is not yet implemented.
   2. Exact Aggregator translation has some pending problems. Details are in this issue:

[codecov]

Codecov Report

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[CunjunWang]

Refactored in #280