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;

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This change is 

[codecov]

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	Coverage Δ
src/worker/health.rs	77.2% <87.5%> (+1.6%)	⬆️

[jonhoo]

Mostly small nits!

[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

[jonhoo]

Looks good now — time to land 🛬