Fix processing error for pause message on agent config topic

[achawla2012]

Proposed changes

• Fix processing error of pause command
• Add test to process message on agent config topic for metric sender plugin.
• Fix bug on commander and metrics_sender plugin.
  • As per https://github.com/nginx/agent/blob/main/sdk/proto/agent.proto#L102, initial_interval, max_interval & max_elapsed_time are in seconds
  • Fixing time.Duration(interval) that returns value in Nanoseconds to time.Duration(interval*int64(time.Second)
    https://go.dev/play/p/WhArj47Rxv_7

Validation through Agent log

time="2023-05-17T16:10:26Z" level=debug msg="metricSenderBackoff in metrics_sender.go with agent config details:<server:<backoff:<initial_interval:60 randomization_factor:0.1 multiplier:1.5 max_interval:60 max_elapsed_time:120 > > >  "
time="2023-05-17T16:10:26Z" level=debug msg="update metric reporter client backoff settings to {InitialInterval:1m0s MaxInterval:1m0s MaxElapsedTime:2m0s Multiplier:1.5 Jitter:0.1}, for a pause command details:<server:<backoff:<initial_interval:60 randomization_factor:0.1 multiplier:1.5 max_interval:60 max_elapsed_time:120 > > > "

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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