Add `context` to `addTelemetryInitializer` callback?

In https://github.com/microsoft/ApplicationInsights-node.js , you can modify the telemetry using `addTelemetryProcessor` which executed with `envelop` and `context` (the context changes according to the collected data, for example: the req/res for HTTP requests).

Any chance we can do the same for this library? :)

Start by locating the addTelemetryInitializer API in this TypeScript SDK and compare it with the addTelemetryProcessor behavior in the linked ApplicationInsights-node.js library. Trace how telemetry context is collected for requests, then determine the callback contract and tests needed to verify that context is available to initializer callbacks.

Written by the indexing model from the issue text.