Log errors that occur during /explain

[plcplc]

What

Make an OpenTelemetry log event when the queries to /explain encounter an error.

How

OpenTelemetry log events are simply events with the field meta.signal_type = "log", and otherwise adhering to the semantic conventions of logs, see https://opentelemetry.io/docs/specs/otel/logs/data-model/.

Similarly, we set error = true, which both Honeycomb and Jaeger recognizes in the UI.

The tracing::event!(..) macro captures the place in the code where it was called. Therefore I have opted to not try and abstract over making error-log events, as that information becomes just noise otherwise.

The body attribute of the event/log is just the Display'd Error message. Depending on the future uses we want for these log messages we will probably amend them with more attributes/ more information.

In Jaeger:

In Honeycomb:

[SamirTalwar]

Are you planning on making the same change to query.rs?

[plcplc]

Are you planning on making the same change to query.rs?

Yes, and potentially other places as described in the issue. I just wanted to have one thing in before I started duplicating it.