metrics: Add a `reason` label to the HS error metrics.

This adds a reason label to the hs_intro_rejected_intro_req_count and hs_rdv_error_count metrics introduced in #40755 (closed).

Metric look up and intialization is now more a bit more involved. This may be fine for now, but it will become unwieldy if/when we add more labels (and as such will need to be refactored).

Also, in the future, we may want to introduce finer grained reason labels. For example, the invalid_introduce2 label actually covers multiple types of errors that can happen during the processing of an INTRODUCE2 cell (such as cell parse errors, replays, decryption errors).

Signed-off-by: Gabriela Moldovan gabi@torproject.org