httpx: add the is_instrumented flag to the transport

[Kludex]

We've found a bug where if the transport is reused in different HTTP clients, the transport gets instrumented multiple times.

Run the following MRE:

from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import SimpleSpanProcessor, ConsoleSpanExporter
from opentelemetry.instrumentation.httpx import HTTPXClientInstrumentor
import httpx

exporter = ConsoleSpanExporter()
span_processor = SimpleSpanProcessor(exporter)
tracer_provider = TracerProvider()
tracer_provider.add_span_processor(span_processor)


transport = httpx.HTTPTransport()
client = httpx.Client(transport=transport)
client2 = httpx.Client(transport=transport)

HTTPXClientInstrumentor().instrument_client(client, tracer_provider=tracer_provider)
HTTPXClientInstrumentor().instrument_client(client2, tracer_provider=tracer_provider)

client.get("https://httpbin.org/get")

You'll see 2 spans.

This PR proposes adding the _is_instrumented_by_opentelemetry to the Transport as well.

[alexmojaki]

We shouldn't log a warning if someone instruments a new client with the same transport. So I think we need to keep the attribute and warning on the client, but silently do nothing if the transport is already instrumented.

[xrmx]

I think the title and description do not match the code, you are moving the flag that signals that the transport has already been instrumented to the transport itself.

[Kludex]

I think the title and description do not match the code, you are moving the flag that signals that the transport has already been instrumented to the transport itself.

Right. The PR started as what the title suggests, and I modified the code given this comment: #3106 (comment)

[Kludex]

@xrmx Should be better now. 🙏

[Kludex]

@xrmx this is not ready, sorry.

I'm missing something, now I have 2 spans on my test, instead of 3 (before my changes were 3). I still need to fix it somewhere.