metricstarttimeprocessor: Implementation of the subtractinitial strategy (open-telemetry#38594)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Addresses open-telemetry#38379.

Some of this logic already existed in the [GCM
exporter](https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/blob/main/exporter/collector/internal/normalization/standard_normalizer.go).

For each resource, and for each timeseries
- Skip if the point is not a cumulative sum, histogram, exp histogram,
or summary.
- 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.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#38379.

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added unit and integration tests.

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#### Documentation
Readme already covers this strategy

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---------

Signed-off-by: Ridwan Sharif <[email protected]>

Author: ridwanmsharif

.chloggen/subtract-strategy-v2.yaml

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+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: metricstarttimeprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Implementation of the subtractinitial strategy
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [38379]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []