Add the ability to initialise the context based on the incoming request and the transaction selected

Add the ability to initialise the context based on the incoming request and the transaction selected. An application may want to change how the Context is constructed based on-

1. the coming request (such as the value of particular http headers)
2. what operation was selected (such as selecting metrics appropriate to that operation)

No files, tests, or entry points are named. Start by tracing how the Context is currently constructed from the incoming request and selected transaction, then determine the API and lifecycle needed to customize it for request headers and operation-specific metrics. Done means the design and implementation support both customization cases with coverage for the resulting behavior.

Written by the indexing model from the issue text.