Fix panic when updating NGINX config and fix remotely enabling/disabling the metrics feature

[bejjrajesh]

Proposed changes

Agent currently has the support to enable/disable features dynamically from NIM. This MR contains change to support dynamically enable/disable metrics feature from NIM based on a flag in nms.conf file. When customer flip the flag to false in nms.conf, we remove metrics and metrics-sender features from payload and send an update to agent, when customer enables flag we add metrics and metrics-sender feature in the payload and send an update to agent.

1. This MR will fix the panic error happen when updating the nginx config concurrently.
   
2. The metrics_sender plugin is updated with access to current configuration and it's updated to set readyToSend flag when metrics and metrics_sender is present in the agent config and set to false when metrics and metrics_sender is not present in the current agent config. These changes are tested with NIM and working properly.

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

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[bejjrajesh]

I have hereby read the F5 CLA and agree to its terms

[dhurley]

@bejjrajesh Can you run the following make commands:

make install-tools
make deps

These will setup some tools, install git hooks and update the vendor folders.

[dhurley]

Closing this PR as further changes are required.
This issue will be resolved by this new PR #1088