rfc: tap-style trace-logging

[siggy]

Summary

Trace-level logging in the proxy is useful for debugging. Currently to achieve this, users redeploy the proxy with logging turned up:
#2703 (comment)

Proposal

Introduce a CLI command that provides trace-level information from a specific proxy:

# linkerd trace [flags] (RESOURCE)
linkerd trace po/pod-foo-bar
linkerd trace -n emojivoto deploy/web

This command could follow a similar model to tap, and would complete the linkerd metrics command nicely.

[grampelberg]

Related to #2005.