metrics: handle values from config file

[manav2401]

Description

The metrics package uses global variables to enable metrics collection (with the flag --metrics and --metrics.expensive). As we're moving towards a toml config based approach, we found out that the values of these 2 parameters from toml file weren't being honoured in some cases. On further investigation we found out that due to the call to init() (in metrics/metrics.go), OS arguments were being read and metrics were being set based on those arguments.

Now, there are some metrics which needs to be registered as soon the bor process starts. We were setting these global values but at much later point in the code when we're done setting up the node related processes which caused all the metric (meters, counters, gauges, etc) registrations to fail and hence leading to 0 values in prometheus. If we were to start bor using cli, it would work as it finds the flags in OS args but not in case of toml.

This PR fixes this by reading the config file itself in the init() function of metrics package and enabling/disabling the global variables before any other process starts.

Changes

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have added at least 2 reviewer or the whole pos-v1 team
• I have added sufficient documentation in code
• I will be resolving comments - if any - by pushing each fix in a separate commit and linking the commit hash in the comment reply

Testing

• I have tested this code manually on local environment
• I have tested this code manually on mumbai/mainnet

[cffls]

LGTM, thanks!

[codecov-commenter]

Codecov Report

Base: 56.83% // Head: 56.86% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (4131b3c) compared to base (aab7214).
Patch coverage: 30.43% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #565      +/-   ##
===========================================
+ Coverage    56.83%   56.86%   +0.02%     
===========================================
  Files          606      606              
  Lines        70128    70149      +21     
===========================================
+ Hits         39860    39890      +30     
- Misses       26845    26852       +7     
+ Partials      3423     3407      -16     

Impacted Files	Coverage Δ
internal/cli/server/server.go	30.21% <0.00%> (-0.68%)	⬇️
metrics/metrics.go	10.95% <36.84%> (+9.17%)	⬆️
p2p/discover/v4_udp.go	72.75% <0.00%> (-4.35%)	⬇️
les/pruner.go	60.71% <0.00%> (-3.58%)	⬇️
les/distributor.go	79.68% <0.00%> (-2.35%)	⬇️
consensus/ethash/ethash.go	51.90% <0.00%> (-2.30%)	⬇️
p2p/discover/table.go	82.48% <0.00%> (-1.83%)	⬇️
core/rawdb/chain_iterator.go	58.38% <0.00%> (-1.25%)	⬇️
rpc/client.go	82.55% <0.00%> (-1.17%)	⬇️
les/costtracker.go	62.43% <0.00%> (-0.53%)	⬇️
... and 15 more

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