schemas/tracing.yml: add span.id

[axw]

Add span.id to the tracing fields. This field may be useful for correlating logs with more specific operations, or where transaction.id is unavailable.

The transaction.id field is not always available for trace data received from other tracing systems, such as Jaeger and OpenTelemetry. We are guaranteed to have trace.id and span.id, but not necessarily transaction.id. Correlating logs with an entire trace (via trace.id) may result in too broad a set of results, compared to correlating with span.id.

[felixbarny]

Does it mean we have to change the query that correlates traces and logs so that we also query for span.id: $transaction.id in the Logs UI?

[axw]

@felixbarny not sure I follow, so I'll expand a bit on the problem.

OpenTelemetry has no concept of transaction; a trace is just a collection of causally-ordered spans. Spans have a "kind", e.g. Server (i.e. server-side of a request) and Client (client-side of a request). We convert Server kind spans into transactions, but there's no reference to the that span's ID in other service-local spans, aside from its children. Even then we have no way of knowing the parent is the local root.

So for OpenTelemetry, the best we can expect to have is:

• trace.id and transaction.id OR
• trace.id and span.id

We probably want to extend the Logs UI to query for span.id: $span.id if that field exists in the log.

[felixbarny]

So far, we only correlate based on the trace.id. For the Trace logs action, I think we wouldn't want to change this.

When jumping from the logs UI to the trace view, we currently also just consider the trace.id.

So I guess we don't need immediate changes. But we'll have to consider this for a potential logs tab in the transaction view.

[webmat]

This LGTM. Thanks everyone :-)