Implement googlemanagedprometheus exporter

[dashpole]

Description:

Fixes #10543

The googlemanagedprometheus exporter is an exporter for Google Cloud Monitoring that uses a custom domain prefix (prometheus.googleapis.com), a fixed monitored resource (prometheus_target), and adds type suffixes to the metric name. Utility functions for mapping to prometheus_target and adding type suffixes are provided by https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/tree/main/exporter/collector/googlemanagedprometheus.

This is implemented by defining the GMPConfig with fewer options than the standard googlecloud exporter, and defining a conversion function to allow using the googlecloud exporter library.

Testing:

Added unit tests, which are modeled after those found in https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/googlecloudexporter

Documentation:

Updated README.md