Add description for custom metrics `tags`

[R-HNF]

What does this PR do?

• Add description for custom metrics tags. This tags is the field that can be configured under custom_queries.
• Also, made some minor fixes to the README.md.

Motivation

There is no description for custom metrics tags in README.md. I think it's necessary to clarify that, unlike the tags configured under instances, the tags configured under custom_queries apply only to custom metrics. Please refer the following: https://github.com/DataDog/integrations-core/blob/e7589a6d452b8d26f9d5e5b650415418d1b46b6c/elastic/datadog_checks/elastic/elastic.py#L510-L612

Minor fixes:

• Remove a trailing space.
• Adjust whitespace in curl commands.
• Use an absolute URL for the link.
  • To prevent redirection to the following URL when accessed from GitHub: https://github.com/DataDog/integrations-core/blob/master/account/settings/agent/latest

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• [ ] If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions[bot]]

⚠️ Recommendation: Add qa/skip-qa label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.