Advanced Observability for H2

Continuation of https://github.com/linkerd/linkerd/issues/1507

Update the stats filter from https://github.com/linkerd/linkerd/issues/1507 to accept an H2ResponseClassifier as a stack param. H2ResponseClassifier is essentially the type:

PartialFunction[(Request, Try[(Response, Try[Frame])]), ResponseClass]

Whenever any of the following conditions are met, the stats filter should consult the classifier and increment the success or failures stat accordingly:

• The response future fails. (req, Throw(e)) should be passed to the classifier.
• The response stream fails. (req, Return(rsp, Throw(e))) should be passed to the classifier.
• The response stream completes successfully. (req, Return(rsp, Return(f))) should be passed to the classifier where f is the final frame (EOS) of the response stream.

[adleong]

That would be “hella dope”. Similarly: histogram of frame sizes.

[adleong]

We should also add some reasonable built-in classifiers:

• Successful iff response and response stream are successful
• Successful if response and response stream are successful and response status is < 500