Resource calls not returning compressed results

[iwysiu]

Core Prometheus returns compressed results for resource calls, while amazon managed prometheus does not. We should compress the responses for amazon managed prometheus as well.

Repro:
Start editing a panel for prometheus/amazon managed prometheus. Click the builder to select the metric, and notice the difference in response size and headers between the values?start=<time>&end=<time> calls. The prometheus call with have content-encoding: gzip in its headers and be smaller for the same resource and time range.

Note: Prometheus compresses the resource call response whether or not enable_gzip is set to true, which may not be correct behavior.