[siddham-jain]

@csmarchbanks

Fixes #535.

Decorating an async def function with Histogram.time(), Summary.time(), Gauge.time(), Counter.count_exceptions() or Gauge.track_inprogress() currently wraps only the creation of the coroutine, not its execution. So the decorators silently record wrong data, e.g. a FastAPI endpoint that awaits for 100ms is observed as ~1µs, raised exceptions are not counted, and the in-progress gauge is back to 0 before the body runs. The wrapper is also no longer a coroutine function, so inspect.iscoroutinefunction() returns False for it.

Changes:

• Backport coroutine support from upstream decorator 4.1 into the bundled decorator.py: FunctionMaker.create generates an async def wrapper with return await when the caller is a coroutine function (and make uses DEF.search so that template parses). Signatures stay preserved as before.
• context_managers.py uses an async caller for coroutine functions, so the with block spans the awaited call. Sync functions take the same path as before.
• Tests for all five decorators on async def functions, including overlapping calls via asyncio.gather, plus a note in the instrumenting docs.

This picks up the direction discussed in #535 now that Python 2 is gone, and supersedes the stalled #767, whose tests cover the same cases.