Added section to enable default metrics through jmxfetch

[domalessi]

What does this PR do?

https://datadoghq.atlassian.net/browse/DOCS-6911 Added instructions for enabling default JBoss/Wildly-DD integration metrics via JMXFetch.

Motivation

JIRA ticket, request from Fred Shad, which I believe was inspired by customer feedback/support requests.

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Changelog entries must be created for modifications to shipped code
• [ ] 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