Add Metrics() to HTTP an GRPC proxies

[roobre]

Description

This PR introduces a new Metrics method to the Proxy interface. The main motivation for this is provide proxies a way to keep track of and record the number of requests they handle, which then can be used to aid in troubleshooting functionality such as #223.

The newly added method is very slim and proxy implementations that do not support metrics can simply return nil.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make test) and all tests pass.
• I have run relevant e2e test locally (make e2e-xxx for agent, disruptors, kubernetes or cluster related changes)
• Any dependent changes have been merged and published in downstream modules