Fix calculation and doc comment in prometheus observer

[CaptainIRS]

• Fixed histogram calculation
• Fixed doc comment in test utility class

[codecov[bot]]

Codecov Report

Merging #1456 (c297544) into main (69000d4) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1456   +/-   ##
=======================================
  Coverage   55.56%   55.56%           
=======================================
  Files         103      103           
  Lines        4451     4451           
  Branches      676      676           
=======================================
  Hits         2473     2473           
  Misses       1435     1435           
  Partials      543      543           

Flag	Coverage Δ
caliper-core	44.31% <100.00%> (ø)
caliper-fabric	80.46% <ø> (ø)
generator-caliper	83.62% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...worker/tx-observers/prometheus-push-tx-observer.js	74.19% <100.00%> (ø)
.../lib/worker/tx-observers/prometheus-tx-observer.js	69.69% <100.00%> (ø)

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