CI should run benchmarks and compare on all PR's

[glightfoot]

Now that we have more benchmarks, it would be good if CI ran benchmarks on all PR's and commented with the result and the benchcmp/benchstat output. Not sure how other projects are doing this

[matthiasr]

We are using the common build infrastructure (Makefile) from github.com/prometheus/common without much extra. I'm willing to give this a try – my main worry is that it makes PRs too noisy to work with but we will see.

Is there prior art for this that we can lean on?

[glightfoot]

It looks like there is some benchmarking stuff in prometheus/test-infra https://github.com/prometheus/test-infra/tree/master/funcbench.

Yeah it could definitely get noisey. Funcbench seems to support using github comments to trigger a run, so that would probably be good enough