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>
