[contrib/net/http] add option to wrap a provided *http.ServeMux

[powerbill]

There are many cases where the traced serve mux should be the 'outer' most http handler in a chain of wrapped handlers. For example, if one wanted to write a middleware which logged all requests by wrapping a http.Handler and they wanted this logger to log dd.trace_id and dd.span_id, that logger would have to execute after the traced http.Handler. One could achieve this using the
contrib/net/http#WrapHandler function, but this loses the nice capability of naming the resource using the http.ServeMux#Handler method because the resourceNamer config does not have access to the mux itself.

Another approach, would be to have the contrib/net/http.ServeMux take another ServeMux as an option, and use that as its mux if provided. This way, the first 'handler' in a chain of request handlers would be the contrib/net/http.ServeMux. This traced mux would add trace context to the http request. This traced request would then be passed to the e.g. logging middleare mux, and so on, and so forth.

What does this PR do?

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!