Changing headers filtering and value setting conditions

[biosvs]

This is an umbrella issue for proposed changes in a headers filtering and value setting conditions.

Currently underlying implementation of headers handling wary significantly from one transport to another (detailed doc will be merged with #2259, docs/docs/headers-handling.md). Eventually, in one of the following releases, we want to make behaviour consistent and protective: filter inbound 'rpc-' and '$rpc$-' headers, return error for attempting of 'rpc-' or '$rpc$-' 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.