Use composable index templates for metric indices

[inqueue]

In this commit, Rally is being switched from using legacy index templates to newer composable templates for metrics indices. Legacy index templates have been deprecated since ES 7.8 in favor of composable templates, and are not supported in ES Serverless. Rally metrics index templates have just one modification to make them composable templates. No other modifications have been made.

• Elasticsearch favors composable templates over legacy index templates when both have matching index patterns. After this change, new metrics indices will use the new index templates.

Closes https://github.com/elastic/rally/issues/1810.

[inqueue]

LGTM.

What do you think about adding a migration guide entry explaining the consequences of this change to existing ES metric stores:

• legacy index templates are not removed automatically but composable index templates take precedence (applies to races, results and metrics),
• index template for annotations only gets created when rally-annotations index is missing, so full migration from legacy to composable would require cumbersome renaming of rally-annotations index, followed by Rally run (esrally add annotation ...), followed by reindexing of old data to new rally-annotations index.

Hi @gbanasiak, the requested changes have been applied. Would you mind taking another look?