tp: allow overriding built-in metrics using extensions

By now specifying
--metric-extensions src/trace_processor/metrics@/
we can now override built-in metrics with fresh ones in the repo
*without* recompiling.

This allows for faster dev cycles when developing metrics.

We're also gating this feature behind the --dev-features flag which
makes sure this doesn't accidentally end up being depended on in
production.

Bug: 190219056
Change-Id: I30d1077844640759093fe231ad2003a6ab1969dd