Span names for flask requests only show the HTTP method if they didn't match a route

Steps to reproduce

1. Instrument a Flask app with FlaskInstrumentor
2. Visit a URL that matches a route and returns 200
3. Visit a different URL that doesn't match a route and returns 404
4. Check the span names for the two requests

Example code:

from flask import Flask
from opentelemetry.instrumentation.flask import FlaskInstrumentor
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import ConsoleSpanExporter, SimpleSpanProcessor

tracer_provider = TracerProvider()
processor = SimpleSpanProcessor(ConsoleSpanExporter(formatter=lambda span: f'!!! Span: {span.name}\n'))
tracer_provider.add_span_processor(processor)

app = Flask(__name__)
FlaskInstrumentor().instrument_app(app, tracer_provider=tracer_provider)


@app.route('/item/<int:item_id>')
def get_item(item_id):
    return f'item {item_id}'


app.run()

Then open in your browser:

• http://127.0.0.1:5000/item/123
• http://127.0.0.1:5000/item/foobar

Note that the second request has the right general pattern but fails to match a route purely because foobar can't be converted to an integer.

What is the expected behavior?

!!! Span: GET /item/<int:item_id>
!!! Span: GET /item/foobar

What is the actual behavior?

The first GET is missing:

!!! Span: /item/<int:item_id>
!!! Span: GET /item/foobar

Additional context

The reason for this is clearly visible in the code. Requests with a matching Flask route don't have the HTTP method included:

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/47caeab7aff47070c565cd90536a39ec5eb06f34/instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py#L295-L300

But if they don't match, they fallback to the WSGI span name which includes the method:

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/47caeab7aff47070c565cd90536a39ec5eb06f34/instrumentation/opentelemetry-instrumentation-wsgi/src/opentelemetry/instrumentation/wsgi/__init__.py#L451-L466

BTW, this PR comment requested removing name_callback which would have allowed customising the span name myself to work around this issue, but I don't see the reasoning behind this.

Start in instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/init.py at the linked span-name logic, then compare it with instrumentation/opentelemetry-instrumentation-wsgi/src/opentelemetry/instrumentation/wsgi/init.py. Verify the matched-route and unmatched-route examples produce consistent HTTP-method-prefixed span names.