Consider unifying span_id with the logfmt event_fields mechanism

[adnandnv]

Context

PR #2354 added an "event fields" mechanism to the logfmt layer: configured span attributes
(e.g. component, object_id) are surfaced onto log lines and inherited from parent spans.
span_id is still handled by separate special-case code and, unlike those fields, is not
inherited in the logfmt path. @Matthias247 suggested it could go through the same mechanism
(PR #2354).

Worth a look

• span_id is a per-work-unit correlation id (set on trace-root spans); its purpose is to
  group every log line of one unit of work ("filter by span_id to see the whole iteration").
• api-core's own log stream / Debug WebUI already inherits it (enclosing_span_id walks the
  span scope), but the logfmt text path does not — so nested lines can drop it.

References

• PR feat(logging): add a component field to log metadata #2354, reviewer note: feat(logging): add a component field to log metadata #2354 (comment)