Added support for endpoints in httpcheckreceiver

[VenuEmmadi]

Description

This PR enhances the httpcheckreceiver by adding support for multiple endpoints (endpoints). Users can now specify a list of endpoints in addition to a single endpoint for each target. This improves flexibility and reduces redundancy when monitoring multiple similar endpoints.

Additional changes include:

• Updates to config.go to handle endpoints.
• Updates to scraper.go to iterate over and scrape all specified endpoints.
• Added unit tests for the new functionality in config_test.go and scraper_test.go.
• Updated documentation (README.md) to reflect the changes.

Link to Tracking Issue

Fixes #37121

Testing

• All existing and new tests pass.
• Tested the httpcheckreceiver manually using the following configuration:

receivers:
  httpcheck:
    collection_interval: 30s
    targets:
      - method: "GET"
        endpoints:
          - "https://opentelemetry.io"
      - method: "GET"
        endpoints: 
          - "http://localhost:8080/hello"
          - "http://localhost:8080/hello"
        headers:
          Authorization: "Bearer eyJhbGciOiJIUzUxMiJ9.eyJzdWIiOiJqYXZhaW51c2UiLCJleHAiOjE3MzcwMzMzMTcsImlhdCI6MTczNzAxNTMxN30.qNb_hckvlqfWmnnaw2xP9ie2AKGO6ljzGxcMotoFZg3CwcYSTGu7VE6ERsvX_nHlcZOYZHgPc7_9WSBlCZ9M_w"   
      - method: "GET"
        endpoint: "http://localhost:8080/hello"
        headers:
          Authorization: "Bearer eyJhbGciOiJIUzUxMiJ9.eyJzdWIiOiJqYXZhaW51c2UiLCJleHAiOjE3MzcwMzMzMTcsImlhdCI6MTczNzAxNTMxN30.qNb_hckvlqfWmnnaw2xP9ie2AKGO6ljzGxcMotoFZg3CwcYSTGu7VE6ERsvX_nHlcZOYZHgPc7_9WSBlCZ9M_w"
processors:
  batch:
    send_batch_max_size: 1000
    send_batch_size: 100
    timeout: 10s
exporters:
  debug:
    verbosity: detailed
service:
  pipelines:
    metrics:
      receivers: [httpcheck]
      processors: [batch]
      exporters: [debug]

Documentation

Describe any documentation changes or additions:

<!-- Describe the documentation added. -->
#### Documentation
- Updated the `README.md` to include examples for `endpoints`.
- Verified `documentation.md` for metric output consistency.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: VenuEmmadi (f7b9cdc, e79a902, ad8b022, a1b9f1f, 09d4c88, 0b98a2d, e7d8ebd, 07d4b2a, 3a6241c, f89ab0a, ea81c9e, 686e4a5, 8c945e0, aec478e, adb84f7)

[VenuEmmadi]

Hi @codeboten and @mx-psi , I hope you're doing well. When you have some time, could you kindly review and share your feedback on this PR? Your input would be greatly appreciated. Thank you!

[atoulme]

LGTM, some nits.

[VenuEmmadi]

Hi @atoulme and @mx-psi, , since this PR is already approved, could you please advise on the next steps to get it merged? Thank you!

[atoulme]

Next step is a review by the codeowners.

[codeboten]

thanks @VenuEmmadi, could the endpoint be marked as deprecated? seems unnecessary to have both, this could be done in a separate PR, not necessary to do it here

[VenuEmmadi]

Hi @mx-psi, We now have the codeowner's approval also. Could you please proceed to merge the code?
Thanks!

[mx-psi]

thanks @VenuEmmadi, could the endpoint be marked as deprecated? seems unnecessary to have both, this could be done in a separate PR, not necessary to do it here

@VenuEmmadi Before we merge this, could you either file an issue for this or open a PR for this?

[VenuEmmadi]

thanks @VenuEmmadi, could the endpoint be marked as deprecated? seems unnecessary to have both, this could be done in a separate PR, not necessary to do it here

Hi @codeboten and @mx-psi Thank you for the suggestion regarding marking endpoint as deprecated. However, since endpoint is part of the confighttp configuration, it cannot be deprecated. The httpcheck receiver relies on confighttp, and the endpoint is used indirectly within the configuration structure.

Removing or deprecating it would disrupt the functionality and standard configuration pattern provided by confighttp. As such, we should continue supporting endpoint alongside the new endpoints feature for now.

[codeboten]

Captured an issue around adding a validation check to prevent users from setting the endpoint field here: #37460