better documentation on metrics

[jkldgoefgkljefogeg]

can use better documentation on how to interpret the metrics

• rpki_client_repository
  • what does type HTTP mean? TA HTTP URI from TAL?
• rpki_client_objects / rpki_client_ta_objects / rpki_client_repository_objects
  • what is counted as valid / failed parse / invalid? e.g. manifest mismatch, object expired, CRL expired
• rpki_client_repository_state
  • 0 failed for RRDP, those with RRDP -> rsync fallback does not count as failure under RRDP which is confusing (which metric reports failures after exhausting all protocols?)