Support for OTEL_PROPAGATORS other than awsxray

[michaelhyatt]

Is your feature request related to a problem? Please describe.
At the moment, Python lambdas only support AWS-XRay trace propagation. Setting OTEL_PROPAGATORS environment variable to anything than awsxray is ignored.

Describe the solution you'd like
Support for other context propagators, such as W3C and B3 by setting OTEL_PROPAGATORS=tracecontext, for example.

Describe alternatives you've considered
I managed to get it to work with tracecontext by cloning the otel-instrumentation script, adding environ["OTEL_PROPAGATORS"] = "tracecontext" and setting AWS_LAMBDA_EXEC_WRAPPER env variable to it.

[anuraaga]

Thanks @michaelhyatt - agree that we should check for the presence of the variable before overwriting it like we do in other languages