Add tracing and examples for `run` and `run_to_completion`

[rustworthy]

This PR is also adding:

• tracing dep and emitting heartbeat event at trace level - so that we know that we are healthy;
• examples run and run_to_completion;

---

This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.0%. Comparing base (a4832db) to head (9974a67).

Additional details and impacted files

| [Files](https://app.codecov.io/gh/jonhoo/faktory-rs/pull/66?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jon+Gjengset) | Coverage Δ | | |---|---|---| | [src/worker/health.rs](https://app.codecov.io/gh/jonhoo/faktory-rs/pull/66?src=pr&el=tree&filepath=src%2Fworker%2Fhealth.rs&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jon+Gjengset#diff-c3JjL3dvcmtlci9oZWFsdGgucnM=) | `77.2% <87.5%> (+1.6%)` | :arrow_up: |

[rustworthy]

@jonhoo Please have a look at this PR once again.

The tests are actually failing for the main branch, which is will be fixed in a dedicated PR: we simply need to adjust a couple of tests to use Client::current_info instead of the retired Client::info.

UPDATE: addressed in #68