metrics: Add support for histograms.

• metrics: Add support for histograms.

  This will enable us to add e.g. circuit build metrics (#40717).

• metrics: Add HS service side circuit build time metrics.

  This adds 2 histogram metrics for hidden services:

  • tor_hs_rend_circ_build_time - the rendezvous circuit build time in milliseconds
  • tor_hs_intro_circ_build_time - the introduction circuit build time in milliseconds

  The text representation representation of the new metrics looks like this:

  # HELP tor_hs_rend_circ_build_time The rendezvous circuit build time in milliseconds
  # TYPE tor_hs_rend_circ_build_time histogram
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="1000.00"} 2
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="5000.00"} 10
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="10000.00"} 10
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="30000.00"} 10
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="60000.00"} 10
  tor_hs_rend_circ_build_time_bucket{onion="<elided>",le="+Inf"} 10
  tor_hs_rend_circ_build_time_sum{onion="<elided>"} 10824
  tor_hs_rend_circ_build_time_count{onion="<elided>"} 10
  # HELP tor_hs_intro_circ_build_time The introduction circuit build time in milliseconds
  # TYPE tor_hs_intro_circ_build_time histogram
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="1000.00"} 0
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="5000.00"} 6
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="10000.00"} 6
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="30000.00"} 6
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="60000.00"} 6
  tor_hs_intro_circ_build_time_bucket{onion="<elided>",le="+Inf"} 6
  tor_hs_intro_circ_build_time_sum{onion="<elided>"} 9843
  tor_hs_intro_circ_build_time_count{onion="<elided>"} 6

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