[wip] metricstarttimeprocessor: add subtractinitial strategy implementation

[ridwanmsharif]

Description

For each resource, and for each timeseries:

• Skip if the point is not a cumulative sum, histogram, exp histogram, or summary, or if it already has a start timestamp [WIP]
• Lookup the timeseries in the cache. If it is not present, drop this point from the batch, and store the timestamp and value (including bucket counts, etc) in the cache.
• Subtract cumulative values (e.g. sum, bucket counts) of the cached timeseries from the current timeseries, and set the start timestamp to the cached point's timestamp.
• Detect resets [wip]

Link to tracking issue

Fixes #38382 and #38379

Testing

Unit tests added for the new strategy [wip]

Documentation

Read me updated [wip]

[ridwanmsharif]

This PR is getting more complicated than I imagined. I'm abandoning it in favor of smaller PRs that do the following:

• Decouple caching from the strategies. This should also fix the reset issue I am facing
• Add implementation of the subtractinitial strategy

[ridwanmsharif]

New attempt at this lives here: #38594

It will be made ready to review once #38579 and #38583 are merged.