Implements OpenTelemetry Integration

[shikokuchuo]

Closes #307.

This PR adds OpenTelemtery support by creating an otel tracing span for top-level functions.

Each existing debug message is then added as an 'event' on the span.

Some questions for @jennybc:

1. Is it 'safe' to log all of these details given the person collecting these can be someone other than the user (e.g. the hash and user email appear just beyond the screenshot cutoff), or do we need to redact some measures.
2. I've assumed that this should just be enabled whenever tracing is enabled and no opt-in is required.
3. token_fetch() is the only instrumented function for now - which other ones should we cover?

Thanks!

To do:

• Update and complete implementation
• Add tests