GMP: target_info and otel_scope_info metrics are INT-typed

[dashpole]

Prometheus metrics scraped from a prometheus endpoint are always double-typed, so if a user is using GMP's managed collection, and has a target_info or otel_scope_info metric, that metric will have a double value type.

This means that using the collector's GMP exporter and managed GMP collection will result in one of the two having write errors.

This is similar to #798, in that we want to write all GMP metrics as double to avoid this scenario.