Add httpx support

[rads-1996]

Description

Add support for httpx instrumentation. Fixes - #46286

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR adds httpx to the set of libraries automatically instrumented by the Azure Monitor OpenTelemetry Distro. It wires httpx into the distro's supported-instrumentation list, adds the opentelemetry-instrumentation-httpx dependency, provides a sample and a smoke test, and registers httpx in the exporter's statsbeat tracking lists. This extends existing HTTP client instrumentation (requests, urllib, urllib3) to also cover httpx.

Changes:

• Add httpx to the distro's _FULLY_SUPPORTED_INSTRUMENTED_LIBRARIES and the opentelemetry-instrumentation-httpx package dependency, plus docs/sample/test coverage.
• Register httpx in the exporter's statsbeat instrumentation lists (introduces a duplicate entry — see review comment).
• Update configuration tests to expect httpx enabled by default.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
azure-monitor-opentelemetry/azure/monitor/opentelemetry/_constants.py	Adds httpx to fully-supported instrumentations (alphabetically placed — correct).
azure-monitor-opentelemetry/setup.py	Adds opentelemetry-instrumentation-httpx dependency.
azure-monitor-opentelemetry/dev_requirements.txt	Adds httpx dev dependency for tests/samples.
azure-monitor-opentelemetry/README.md	Documents the httpx instrumentation and links.
azure-monitor-opentelemetry/samples/README.md	Lists the new httpx sample.
azure-monitor-opentelemetry/samples/tracing/http_httpx.py	New sample instrumenting httpx.
azure-monitor-opentelemetry/samples/tracing/instrumentation_options.py	Adds httpx to the instrumentation options sample.
azure-monitor-opentelemetry/tests/instrumentation/test_httpx.py	New smoke test for HTTPXClientInstrumentor.
azure-monitor-opentelemetry/tests/utils/test_configurations.py	Updates expected config to include httpx enabled.
azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Adds httpx to statsbeat lists; introduces a duplicate httpx in _INSTRUMENTATIONS_LIST.

Additional note (not attachable to a changed line): neither azure-monitor-opentelemetry/CHANGELOG.md nor azure-monitor-opentelemetry-exporter/CHANGELOG.md has an entry under the Unreleased ### Features Added section for this new httpx support, and the PR checklist flags CHANGELOG as not updated. Consider adding a changelog entry before release.

[JacksonWeber]

LGTM apart from the copilot comment.

[rads-1996]

LGTM apart from the copilot comment.

@JacksonWeber I have addressed the copilot feedback and removed the duplicate declaration.