chore: add basic metrics for rendezvous

[vpavlin]

Since we have enabled Rendezvous Discovery in nwaku by default (as long as relay is enabled), we need to be able to measure some basic usage.

This is my attempt to add basic set of metrics:

libp2p_rendezvous_register - counter for register/advertise requests
libp2p_rendezvous_discover - counter for discover/request requests
libp2p_rendezvous_registered - gauge representing number of currently registered peers
libp2p_rendezvous_namespaces - gauge representing number of currently known namespaces

I am not sure if my approach to setting these metrics is optimal, so would love any suggestions for improvements

[codecov]

Codecov Report

Merging #905 (583b89d) into unstable (c76d1e1) will increase coverage by 0.03%.
The diff coverage is 83.33%.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #905      +/-   ##
============================================
+ Coverage     84.27%   84.30%   +0.03%     
============================================
  Files            91       91              
  Lines         15012    15017       +5     
============================================
+ Hits          12651    12660       +9     
+ Misses         2361     2357       -4     

Impacted Files	Coverage Δ
libp2p/protocols/rendezvous.nim	91.37% <83.33%> (+0.07%)	⬆️

... and 5 files with indirect coverage changes