Include http.target in flask instrumentation metrics attributes

[ElementalWarrior]

trafficstars

As part of the semantic conventions, http.target should be included in the metrics attributes. However it is only present for span attributes.

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/semantic_conventions/http-metrics.md#parameterized-attributes

Currently it isn't part of the duration_attrs. Which it probably shouldn't until there is a solution to set the value to the flask url_rule instead of the raw input request:

• caller: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/init.py#L295
• callee: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-wsgi/src/opentelemetry/instrumentation/wsgi/init.py#L236