Enhanced metrics for Onion Service descriptors

Implement additional metrics for Onion Service descriptors.

That may need:

• A better way to parse descriptors would enable many other metrics.
• Some patches sent upstream to Stem.

Some fields that could get measurements:

• From the outer descriptor wrapper:
  • "descriptor-lifetime".
  • "revision-counter".
• From the first layer of encryption:
  • "caa-critical". Moved to #84.
• From the second layer of encryption:
  • "caa". Moved to #84.
  • "single-onion-service".
  • "pow-params": an indirect way to measure DoS for PoW-enabled services (by measuring the PoW settings in the descriptor), which depends on tpo/core/tor#40634 (closed) to be implemented.

Other measurements:

• Metrics for the descriptor and inner layer sizes.
• Per-HSDir latency.