Hide auto-populated prometheus_url from config spec

[florimondmanca]

What does this PR do?

Hide the prometheus_url option from Amazon MSK example config.

Motivation

Customer and SE were confused about this option being present and marked as "required", but they didn't know what to put there.

Turns out the option is irrelevant to the MSK check:

integrations-core/amazon_msk/datadog_checks/amazon_msk/amazon_msk.py

Lines 33 to 34 in 2fdc311

	instance = self.instance.copy()
	instance['prometheus_url'] = 'necessary for scraper creation'

The check iterates through MSK nodes returned by the AWS API, and auto-fills the prometheus_url for each:

integrations-core/amazon_msk/datadog_checks/amazon_msk/amazon_msk.py

Lines 78 to 82 in 2fdc311

	for endpoint in broker_info['Endpoints']:
	for (port, metrics_mapper, type_overrides) in self._exporter_data:
	self._scraper_config['prometheus_url'] = '{}://{}:{}/metrics'.format(
	self._endpoint_prefix, endpoint, port
	)

As our test setup confirms, only cluster_arn is really required:

integrations-core/amazon_msk/tests/conftest.py

Lines 34 to 39 in 1bf1bcf

	@pytest.fixture
	def instance():
	return {
	'cluster_arn': 'arn:aws:kafka:us-east-1:1234567890:cluster/msk-integrate/9dabe192-8f48-4421-8b94-191780c69e1c',
	'tags': ['test:msk'],
	}

So we should drop prometheus_url from the docs.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached