Add TxObserver for Prometheus manager

[CaptainIRS]

In this PR:

• The new TxObserver class is added (with tests) as a part of the ongoing effort to add a Prometheus scrape server to the Caliper manager.

PR 1/n for #1353

[codecov[bot]]

Codecov Report

Merging #1448 (4fc70d1) into main (e504bee) will increase coverage by 0.33%. The diff coverage is 82.14%.

@@            Coverage Diff             @@
##             main    #1448      +/-   ##
==========================================
+ Coverage   55.56%   55.89%   +0.33%     
==========================================
  Files         103      105       +2     
  Lines        4451     4507      +56     
  Branches      676      688      +12     
==========================================
+ Hits         2473     2519      +46     
  Misses       1435     1435              
- Partials      543      553      +10     

Flag	Coverage Δ
caliper-core	44.98% <82.14%> (+0.67%)	:arrow_up:
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
packages/caliper-core/lib/common/config/Config.js	73.91% <ø> (ø)
...ker/tx-observers/prometheus-manager-tx-observer.js	80.76% <80.76%> (ø)
...r-core/lib/common/messages/workerMetricsMessage.js	100.00% <100.00%> (ø)

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