HTTP transition for flask and wsgi

[lmolkova]

Built on top of #1987

Server part of #1986

Changes and observations :

• For the 'old' default mode:
  • removes alternative attribute sets for host/url, unifying everything to net.host.name, net.host.port
  • fixes some minor bugs (span name, unknown HTTP method names)
• Implements new "http" and "http/dup" modes
  • The approach in Requests instrumentation http semantic convention migration #2002 (attribute mapping) is not maintainable for server instrumentation:
    • attributes have different req levels for metrics and spans
    • it's really hard to apply conversion at the right time for the right signal

Didn't see any technical challenges or limitations when implementing the new version of spec (except tons of boilerplate code)