Fix duplicate metrics bug

@amalakar @tianx2 Now that the dust has settled from the merging, I did some integration testing in-house and found a bug where `withExpansions` caused duplicate metrics to be pushed to datadog. I added a test to verify and fixed the bug in this PR. I also:

* Added debug-level logging to dump/time HTTP requests to datadog
* Renamed P50 to MEDIAN to be consistent with `metrics` naming
* Did some refactoring in the reporter to reduce verbosity

Let me know if this looks reasonable. Thanks!