Use runtime metrics / new UI

[arl]

Use runtime/metrics as source of metrics (rather than runtime.Memstats which is less performant and less complete).

Rework UI

[codecov[bot]]

Codecov Report

Merging #75 (a072a6b) into main (583356d) will decrease coverage by 8.26%. The diff coverage is 74.19%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   97.91%   89.65%   -8.27%     
==========================================
  Files           3        3              
  Lines          48       87      +39     
==========================================
+ Hits           47       78      +31     
- Misses          1        6       +5     
- Partials        0        3       +3     

Impacted Files	Coverage Δ
statsviz.go	68.18% <57.14%> (-21.82%)	:arrow_down:
handlers.go	94.28% <88.23%> (-5.72%)	:arrow_down:
register.go	100.00% <0.00%> (ø)

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