Add a Prometheus metric reader

[duncanpo]

Currently the only supported metric reader is PeriodicExportingMetricReader, which pushes metric to the OpenTelemetry collector with the help of an Otlp exporter. We should also support the pull model where we expose the metrics at an end point for a backend to scrape.

[duncanpo]

Observations from initial experiments:

1. Prometheus reader/exporter needs to be explicitly enabled when building in otel-cpp by defining WITH_PROMETHEUS
2. Requires prometheus-cpp, which can be available in otel-cpp via submodules.
3. The options without_units and without_type_suffix currently don't seem to be functional.