Don't record GeneratorExit errors in stream RPC spans

[cbrewster]

Why

Stream RPCs which are not read to completion would show up with a GeneratorExit error in their associated span. There are valid cases where a caller may not need to exhaustively read the stream, so these should not be considered errors.

What changed

• Stop using the span as a context manager, we'll handle recording errors ourselves
• Catch RiverExceptions, Exceptions, and CancelledErrors
• Switch from AsyncIterator to AsyncGenerator for stream return types
  • This allows us to call close on the generator
  • We may also want to do this in the codegen, this will be especially useful for cancellation support (just close the generator)

Test plan

• Added a test which ensured a closed generator creates a span with OK status

[cbrewster]

Current dependencies on/for this PR:

• main:
  • PR Don't record GeneratorExit errors in stream RPC spans #129 👈

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