formatter: add SPAN_ID access log formatter

[bittoby]

Commit Message: formatter: add SPAN_ID access log formatter

Additional Description:
Adds a %SPAN_ID% access log command operator that returns the span ID of the active
(downstream) span for a request. This complements the existing %TRACE_ID% formatter
and enables users to correlate access log entries with specific spans in their distributed
tracing backend.

The implementation mirrors TraceIDFormatter exactly - it calls getSpanId() on the
active span from the formatter context. The getSpanId() method already exists on the
Tracing::Span interface and is implemented by several tracing providers (OpenTelemetry,
Fluentd). Providers that have not yet implemented it (Zipkin, Datadog, SkyWalking, X-Ray)
return an empty string, which the formatter handles gracefully by outputting "-".

The original issue also suggested separate %CLIENT_SPAN_ID% / %SERVER_SPAN_ID%
formatters. However, only the downstream (server) span is accessible in the access log
formatter context - the upstream (client) span lives inside the router and is not plumbed
to the log context. A single %SPAN_ID% is the correct approach given the current
architecture. Exposing the upstream span would require deeper changes and can be tracked
separately if needed.

Risk Level: Low
Testing: Added unit tests covering all three code paths: valid span ID present, no active
span set, and span present but returning empty ID (providers that don't implement it yet).
Docs Changes: Added %SPAN_ID% documentation to
docs/root/configuration/advanced/substitution_formatter.rst, placed directly after the
existing %TRACE_ID% entry.
Release Notes: Added entry to changelogs/current.yaml under the formatter area.
Platform Specific Features: N/A
Fixes #43674

[repokitteh-read-only]

Hi @bittoby, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #43997 was opened by bittoby.

see: more, trace.

[bittoby]

@agrawroh @basvanbeek Could you please review this PR? Welcome to any feedbacks
thank you