[1/3 header changes][metrics + grpc] Emit metrics for new headers handling behaviour. Hide actual changes under feature flag.

[biosvs]

Currently underlying implementation of headers handling wary significantly from one transport to another (docs/docs/headers-handling.md in this PR). Eventually, in one of the following releases, we want to make behaviour consistent and protective: filter inbound 'rpc-' headers, return error for attempting of 'rpc-' header setting.

Proposed changes are backward incompatible, so to identify the edges that are affected by the future changes, at first stage let's emit metrics.

Metrics: reserved_headers_stripped and reserved_headers_error metrics with "component": "yarpc-header-migration" constant tag and with source and dest variable tags

Part of https://github.com/yarpc/yarpc-go/issues/2265