Linkerd-proxy expose in-flight requests

## Feature Request

Right now `linkerd-proxy` will expose metrics under `/metrics` api. There is no metric like in-flight request in the metrics it currently exposes.

### What problem are you trying to solve?

We are trying to implement an autoscaling solution based on in-flight request.

### How should the problem be solved?

Adding in-flight metrics to `/metrics` api.

If you could point me the code that need to be added, I am happy to make a contribution :-)

Start by locating linkerd-proxy's existing /metrics endpoint and metric definitions. Determine the intended in-flight request metric semantics from the proxy's request handling, then add the metric and verify that it is exposed through /metrics.

Written by the indexing model from the issue text.