Don't compile in direct calls to callback functions

[GregMefford]

When compiling a project that includes spandex_phoenix (in the example below, the spandex_example project), it throws a compiler warning because we're compiling-in a direct function call on a module in the application using us as a library (which we obviously don't depend on from spandex_phoenix).

Using apply gets around this because it's dynamically finding and calling the function at runtime.

==> spandex_phoenix
Compiling 1 file (.ex)
warning: function PhoenixBackend.Tracer.finish_span/0 is undefined (module PhoenixBackend.Tracer is not available)
Found at 2 locations:
  lib/spandex_phoenix/instrumenter.ex:16
  lib/spandex_phoenix/instrumenter.ex:24

warning: function PhoenixBackend.Tracer.start_span/2 is undefined (module PhoenixBackend.Tracer is not available)
Found at 2 locations:
  lib/spandex_phoenix/instrumenter.ex:12
  lib/spandex_phoenix/instrumenter.ex:20

[GregMefford]

Also, this would be a good learning opportunity for how I'm supposed for format my commit messages and stuff. 😅

[zachdaniel]

For this one, something like:

fix: Indirectly call trace function to remove warning

More in depth description here optionally, but just the first line is fine.

If you can shorten the message enough, you could also use a scope like fix(Instrumenter): did the thing