feat: add logger for use with tracing middleware

[lleadbet]

In #3506, it was discussed to allow users to bring a custom logger to print out the various error logs that might happen during the tracing process to allow users to properly set the right structured logging consistently across the application.

This adds a new Logger attribute on the Tracer struct that can take in a Logger that abides by the standard log library standard (that is it has at a minimum has the Print/ln/f functions).

Defaults to a no-op logger.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 73.398% (-0.09%) from 73.492%
when pulling 19f442f on lleadbet:feat/log-hook-tracing
into 227a886 on 99designs:master.

[StevenACoffman]

Hey, thanks for the quick turn around. I have a few nits, but this looks great.

[lleadbet]

Agreed on both points; had considered the package name (and New function) but was semi-concerned about folks importing that on accident thinking it was related to something else, not the tracing package, but given the import path including the tracing piece it was probably less important.

[StevenACoffman]

Thanks again for coming back to do this! I think this will make it a lot easier to debug if people have problems, and they won't be constrained to a specific logger.

I already cut a new release, so I'd like to wait a few days and accumulate some more stuff before making a new one.

[lleadbet]

no rush on the new release, the actual needed feature was done and this is just a polishing piece :)