[WIP] Add Flask enrich_metric_attributes if Labeler contains custom attributes

[tammy-baylis-swi]

Description

Updates Flask instrumentor to use Labeler enrich_metric_attributes before adding/recording each metric, old and new semconv: HTTP server duration, active requests counter. Has no effect if Labeler is not called by instrumented code or custom distro.

Depends on

• introduction of Labeler in (Redo) Add experimental Labeler to store custom attributes in OTel Context #4288
• enrichment by WSGI/ASGI in [WIP] Add WSGI, ASGI enrich_metric_attributes for HTTP server duration/request/response metrics #4300

Fixes # (issue)

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?

• tox -e py312-test-instrumentation-flask-0
• Install FlaskInstrumentor and manually instrument Flask app with http/dup opt-in; use Labeler to add attrs such as user_id

Example app-side call to add attributes with Labeler:

@app.route("/users/<user_id>/")
def get_users(user_id):
    # Get the labeler for the current OTel context
    labeler = get_labeler()
    # Add custom attributes to Flask instrumentation metrics
    labeler.add("user_id", user_id)
    labeler.add("user_type", "registered")
    # Or, add multiple attributes at once
    labeler.add_attributes(
        {"feature_flag": "new_ui", "experiment_group": "control"}
    )
    return f"User profile for {escape(user_id)}"

Example otelcol debug HTTP server duration metrics from instrumentation

InstrumentationScope opentelemetry.instrumentation.flask 0.62b0.dev
...
Metric #1
Descriptor:
     -> Name: http.server.duration
     -> Description: Measures the duration of inbound HTTP requests.
     -> Unit: ms
     -> DataType: Histogram
     -> AggregationTemporality: Cumulative
HistogramDataPoints #0
Data point attributes:
     -> http.method: Str(GET)
     -> http.server_name: Str(127.0.0.1)
     -> http.scheme: Str(http)
     -> net.host.name: Str(0.0.0.0:8000)
     -> http.host: Str(0.0.0.0:8000)
     -> net.host.port: Int(8000)
     -> http.flavor: Str(1.1)
     -> http.status_code: Int(200)
     -> http.target: Str(/users/<user_id>/)
     -> user_id: Str(123)
     -> user_type: Str(registered)
     -> feature_flag: Str(new_ui)
     -> experiment_group: Str(control)

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

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