Use tracing library to generate spans

[joshwlewis]

Alternative to #917 -- personally, I prefer this one. This sets up tracing so that it emits trace data into our opentelemetry file exporter pipeline while correctly honoring the cascade of events from kodon, through libcnb, and buildpack specific spans.

Using tracing for recording span data in CNBs looks like this:

{
  // This span will be nested within libcnb.rs-generated span
  let _span_guard = tracing::span!(Level::INFO, "download-go", version = go_version).entered();
  do_the_download();
  // This event will be affiliated with the 'download-go' span
  tracing::event!(Level::INFO, "downloaded");
}
// The _span_guard is out of scope, so the 'download-go' span is ended

tracing is a widely used library, even within libraries. This seems like the more rust-like approach. #917 by comparison is the more opentelemetry-like approach and closer to that standard.

As an added bonus the tracing library automatically adds code.lineno, code.filepath, code.namespace attributes to spans and events.

There are some changes to the shape of the tracing data - some span names, some attributes have changed. Most of this is because we are accepting tracing-opentelemetry conventions.

[colincasey]

Ooh, I like this more than #917.

[edmorley]

Thank you! I really like the approach of using the tracing crate - it seems this will be easier for individual language CNBs to adopt :-)