Take accessToken from context in signalfxexporter

[asreehari-splunk]

Description

In preparation for deprecating access_token_passthrough from the signalfx receiver and exporter, update the signalfx exporter to taken accessToken from the context

Testing

• Updated unit tests
• tested end to end with the following setup
  • telemetrygen sending metrics in otlp over grpc with X-Sf-Token header and token
  • otlp receiver with include_metadata set to true
  • batch processor using the X-Sf-Token key to group
  • access_token_passthrough set to true

Documentation

[crobert-1]

Based on the title of this PR starting with Draft:, I've converted this PR to a draft PR. Feel free to update as you see fit. 👍

[dmitryax]

lint and tests are failing

[asreehari-splunk]

@dmitryax ... updated. Lint failure is unrelated to my change. I will look again at the end of this run

[dmitryax]

Please rebase

[dmitryax]

@asreehari-splunk it needs another rebase unfortunately