Add HTTP middleware to rule-evaluator

[maxamins]

This code accomplishes three goals:

1. Remove grpc measurements since grpc is not used by the rule-evaluator.
2. Expose the export metrics collected by export.go. In the future, we may want to measure latencies of CreateTimeSeries.
3. Wrap Query requests sent to GCM API with an instrument RoundTripper to count return codes and measure latencies. This will give GMP more control over their SLO's and Alerts.

This PR will resolve b/300157911

[bwplotka]

Looks solid & simple!

Thanks! 👍🏽