Migrate away from prometheus untyped feature gate

[dashpole]

The upstream prometheus receiver now supports storing the original promethues type in the metric metadata (open-telemetry/opentelemetry-collector-contrib#32605), which is released in v0.100.0 of opentelemetry-collector-contrib.

The googlemanagedprometheus exporter also supports reading the original type from metric metadata, and double-writing it in the GMP format for unknown metrics (GoogleCloudPlatform/opentelemetry-operations-go#839), which is released in v0.47.0 of opentelemetry-operations-go.

This replaces the usage of the gcp.untypedDoubleExport feature gate, and should remove the need to maintain a fork of the prometheus receiver here: https://github.com/GoogleCloudPlatform/opentelemetry-operations-collector/tree/master/receiver/prometheusreceiver

@ridwanmsharif