* Logging functionality for spans.

* Jaeger spans for availability distribution.

* Fix instrumentation to use log target properly.

* Add some tracing instrumentation macros.

* Use int_tags instead of logs.

* Add span per iteration.

* Remove span::log functionality.

* Fix instrumentation log target for real.

* Add jaeger span to responding side as well.

* Revert "Fix instrumentation log target for real."

This reverts commit e1c2a2e6ff6f257e702f07d8a77c2668af92b0ef.

* Revert "Fix instrumentation to use log target properly."

This reverts commit 7caa0bd1acc6fe9727bb3a91851560d756c40ab8.

* target -> subsystem in instrumentatio macro

target is not correct either, and the correct way of using a top level
target = LOG_TARGET does not work, as the macro expects a string literal
and gets confused by the constant `LOG_TARGET`.

* Use kebab-case for spa names.

Co-authored-by: Andronik Ordian <write@reusable.software>

Co-authored-by: Andronik Ordian <write@reusable.software>