  o Minor features (metrics):
    - Add support for histograms.
      Part of ticket 40757.
  o Minor features (hs, metrics):
    - Add tor_hs_rend_circ_build_time and tor_hs_intro_circ_build_time
      histograms to measure hidden service rend/intro circuit build time
      durations.
      Part of ticket 40757.
