Add data streams monitoring instrumentation for HTTP

[hokitam]

What Does This Do

This change adds data streams monitoring instrumentation for HTTP client and server libraries.

For HTTP server libraries, we are able to make the change directly in a single location: in HttpServerDecorator, where the trace context extract() call is made today. To avoid changing the method signature of extract(), we make our call in startSpan instead.

For HTTP client libraries, we add an additional injectPathwayContext() call wherever we make an inject() call today.

Motivation

Support HTTP for Data Streams Monitoring.

Additional Notes

This change was tested manually with demo apps using Apache HTTP client and Java Servlet deployed via Tomcat. Unit tests were added via HttpClientTest and HttpServerTest, which are used as the base tests for the HTTP libraries that the Java tracer supports.

[mcculls]

General comment: it's unfortunate that injecting the pathway context couldn't have been made part of the existing propagate().inject(...) call because that would have meant fewer changes to the codebase.

Even a method that did both such as propagate().injectWithPathway(span, request, SETTER, pathwayTags) would avoid having all these extra method calls per-request.

[mcculls]

Looks ok, given the limitations of the current pathway context API

(there appears to be a lot of opportunity in that code to revisit the API and reduce overhead)