Add dynamic headers to Exporter constructor

[DylanRussell]

The draft PR prototypes adding additional_dynamic_headers to OTLP gRPC / HTTP Exporter constructor.

Additional dynamic headers is a map where the key is the header name and the value is a generator function which produces the header value when called. These header functions are called on each call to export.

Google needs something like this in order to set the Authorization header, which periodically needs to be refreshed and updated.

Here's how we'd use this new constructor:

from opentelemetry.exporter.otlp.proto.http._log_exporter import OTLPLogExporter as HTTPOTLPLogExporter
from opentelemetry.exporter.otlp.proto.grpc._log_exporter import OTLPLogExporter as GRPCOTLPLogExporter
import google.auth
from google.auth.transport import requests

def build_google_grpc_otlp_log_exporter() -> GRPCOTLPLogExporter:
    return GRPCOTLPLogExporter(additional_dynamic_headers={'Authorization': build_auth_stuff})

def build_google_http_otlp_log_exporter() -> HTTPOTLPLogExporter:
    return HTTPOTLPLogExporter(additional_dynamic_headers={'Authorization': build_auth_stuff})

def build_auth_stuff() ->  Iterator[str]:
    credentials, _ = google.auth.default()
    request = google.auth.transport.requests.Request()
    while True:
        if credentials.expired:
            credentials.refresh(request)
        yield f'Bearer: {credentials.token}'

Java has something similar

[linux-foundation-easycla]

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• ✅ login: DylanRussell (718c334)

[Kludex]

Same comment as #4432.

[dimaqq]

Sure, but why a generator and not a callable?

[dimaqq]

Other projects solved this issue, aiodynamo is one such case that I'm familiar with and can recommend looking into for inspiration:

Custom credentials: https://aiodynamo.readthedocs.io/en/latest/advanced.html#custom-credentials-loader

Overall use: https://aiodynamo.readthedocs.io/en/latest/quickstart.html