opentelemetry-api: implement metrics bind API

[herin049]

Description

Implements the OTel metrics Bind API.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

uv run tox -e py314-test-opentelemetry-api

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[Copilot]

Pull request overview

This PR adds support for “bound” synchronous metric instruments (per the OTel Metrics Bind concept), allowing instruments to be pre-associated with a set of attributes and to merge those attributes with per-call attributes at recording time.

Changes:

• Adds _bind() to synchronous instrument types and introduces bound-instrument wrappers that delegate recording while merging/deepcopying attributes.
• Adds API-level tests covering delegation behavior, attribute merging semantics, deepcopy behavior, rebinding flattening, and proxy instrument interaction.
• Adds SDK-level tests validating that bound recordings flow through the measurement consumer and preserve existing validation behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
opentelemetry-api/src/opentelemetry/metrics/_internal/instrument.py	Adds _bind() and bound instrument wrapper implementations for synchronous instruments.
opentelemetry-api/tests/metrics/test_instruments.py	Adds API tests for bound instrument semantics (merge/deepcopy/rebind/proxy).
opentelemetry-sdk/tests/metrics/test_instrument.py	Adds SDK tests to ensure bound instruments propagate measurements/validation correctly.

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