Use timeout flag for context cancelation

[lucacome]

Proposed changes

This pull request introduces several changes to the NGINX Prometheus exporter to add context and timeout handling capabilities. The main updates include modifying the GetStubStats method to accept a context parameter, adding a timeout field to the NginxCollector and NginxPlusCollector structs, and updating the Collect methods to use context with timeout.

Context and Timeout Handling Enhancements:

• client/nginx.go: Modified the GetStubStats method to accept a context.Context parameter, enabling more flexible request handling.

• collector/nginx.go:

  • Added context and time imports to support context with timeout.
  • Added a timeout field to the NginxCollector struct and updated the NewNginxCollector function to accept a timeout parameter.
  • Updated the Collect method to use a context with timeout for fetching stub stats.

• collector/nginx_plus.go:

  • Added time import to support context with timeout.
  • Added a timeout field to the NginxPlusCollector struct and updated the NewNginxPlusCollector function to accept a timeout parameter. [1] [2] [3] [4]
  • Updated the Collect method to use a context with timeout for fetching NGINX Plus stats.

Code Cleanup:

• exporter.go:
  • Simplified the registerCollector function signature and removed the Timeout field from the http.Client configuration. [1] [2] [3]

Closes #858

Checklist

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

• [ ] I have read the CONTRIBUTING guide
• [ ] I have proven my fix is effective or that my feature works
• [ ] I have checked that all unit tests pass after adding my changes
• [ ] I have ensured the README is up to date
• [ ] I have rebased my branch onto main
• [ ] I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 3.89%. Comparing base (9cd3941) to head (487aac4).

Files with missing lines	Patch %	Lines
collector/nginx.go	0.00%	5 Missing :warning:
collector/nginx_plus.go	0.00%	5 Missing :warning:
exporter.go	0.00%	3 Missing :warning:
client/nginx.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #868 +/- ## ======================================== - Coverage 3.90% 3.89% -0.01% ======================================== Files 5 5 Lines 1307 1310 +3 ======================================== Hits 51 51 - Misses 1256 1259 +3 ```

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