Support global scoped histograms that only report aggregate values at global tier.

[clin-stripe]

Currently, histograms only support local and mixedscope. Local scope sends all histogram values from local, forwarding veneurs.

Mixed scopes are more complicated: aggregate values like min, max, count, etc are sent by local veneurs but percentiles are aggregated globally.

To reduce metric cardinality and volume, we want to support globally scoped histograms for which all values are aggregated and reported by the global veneur.

This diff accomplishes that by adding a concept of global histograms inside veneur and the metric protobuf so the scope can be threaded to the global veneur.

Of note, it is necessary to add a hacky flag to the Histogram's Flush method indicating whether we want to flush the locally accumulated aggregate values or those from the globally aggregated histogram. We also had to add support to the tdigest data structure to support global merging.

Motivation

• Necessary for Force all metrics to be sent to the global aggregator #565.
• Metric cleanup efforts at Stripe.

Test plan

1. Additional test coverage for the changes in forwarding and histogram flushing behavior.
2. Set up a mock global veneur tier configuration using the two additional yaml files being checked in.
3. QA staging environment.
4. Canarying in prod.

[sjung-stripe]

implementation looks great to me, nice work. failing test:

05:31:10 --- FAIL: TestLocalServerMixedMetrics (0.01s)
05:31:10     <autogenerated>:1: 
                          
	Error Trace:	server_test.go:406
05:31:10         
	Error:      	Not equal: 
05:31:10         
	            	expected: &tdigest.MergingDigest{compression:100, mainCentroids:[]tdigest.Centroid{tdigest.Centroid{Mean:1, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:2, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:7, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:8, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:100, Weight:1, Samples:[]float64(nil)}}, mainWeight:5, tempCentroids:[]tdigest.Centroid{}, tempWeight:0, min:1, max:100, reciprocalSum:0, debug:false}
05:31:10         
	            	actual  : &tdigest.MergingDigest{compression:100, mainCentroids:[]tdigest.Centroid{tdigest.Centroid{Mean:1, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:2, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:7, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:8, Weight:1, Samples:[]float64(nil)}, tdigest.Centroid{Mean:100, Weight:1, Samples:[]float64(nil)}}, mainWeight:5, tempCentroids:[]tdigest.Centroid{}, tempWeight:0, min:1, max:100, reciprocalSum:1.7778571428571428, debug:false}

this test contains a hardcoded gob, so it'll have to be adjusted. as for other tests:

• e2e tests to make sure forwarding works with the new behavior (i can show you what these look like if you need a hand)
• test to make sure old gobs can still be decoded without error

you'll also need to rebase because this repo is configured for squashes only

[stripe-ci]

Gerald Rule: Copy Observability on Veneur, Unilog, Falconer pull requests

cc @stripe/observability
cc @stripe/observability-stripe

[clin-stripe]

Instead of writing new E2E tests, I wrote these unit tests to validate this behavior change. The other behavior changes (forwarding, mainly) were covered by adding new assertions.

[sjung-stripe]

GOOD SHIT

[sjung-stripe]

ah, looks like it needs gofmt

[sjung-stripe]

^ two spaces after MetricScope here