Elastic: snapshots (slm_stats) not working with OpenSearch

[plumdog]

When targeting OpenSearch and setting slm_stats: true, get an error like:

401 Client Error: Unauthorized for url: https://[myopensearch].es.amazonaws.com:443/_slm/policy"

Traceback (most recent call last):
  File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/datadog_checks/base/checks/base.py", line 1224, in run
    self.check(instance)
  File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/datadog_checks/elastic/elastic.py", line 175, in check
    policy_data = self._get_data(slm_url)
                  ^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/datadog_checks/elastic/elastic.py", line 338, in _get_data
    resp.raise_for_status()
  File "/opt/datadog-agent/embedded/lib/python3.11/site-packages/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: Unauthorized for url: https://[myopensearch].es.amazonaws.com:443/_slm/policy

It looks like OpenSearch doesn't have the the _slm/policy API https://www.elastic.co/guide/en/elasticsearch/reference/current/slm-api-get-policy.html but rather a _plugins/sm/policies API https://opensearch.org/docs/latest/tuning-your-cluster/availability-and-recovery/snapshots/sm-api/. It looks to me like these are logically similar, but OpenSearch doesn't provide the same stats values as Elasticsearch to provide the slm metrics.

However, there is the _snapshot/ API that both support (see https://www.elastic.co/guide/en/elasticsearch/reference/current/get-snapshot-api.html and https://opensearch.org/docs/latest/api-reference/snapshots/get-snapshot/) that could be used to provide useful metrics, eg the number of snapshots for each index, the oldest and most recent snapshots for each index.