Expose SpanID and TraceID in workflow context

[cschleiden]

Problem

Fixes #473 — Users cannot obtain the TraceID and SpanID from within workflow code. After calling workflow.Tracer(ctx).Start(ctx, "name"), the returned Span only exposes End(), making it impossible to access trace context. Additionally, workflow.Logger(ctx) doesn't include trace correlation attributes.

Changes

1. SpanContext() on workflow.Span interface

The Span interface now exposes SpanContext() trace.SpanContext, allowing users to retrieve TraceID and SpanID:

ctx, span := workflow.Tracer(ctx).Start(ctx, "my operation")
defer span.End()

traceID := span.SpanContext().TraceID()
spanID := span.SpanContext().SpanID()

2. workflow.SpanFromContext(ctx)

New public helper to get the current active span from the workflow context (mirrors OTel's trace.SpanFromContext):

span := workflow.SpanFromContext(ctx)
if span != nil {
    traceID := span.SpanContext().TraceID()
}

3. Trace context in workflow logger

The workflow logger now automatically includes trace_id and span_id as structured log attributes, so all log entries from workflow.Logger(ctx) are correlated with the workflow's trace span.

Files changed

• workflow/tracer.go — Added SpanContext() to interface + SpanFromContext function
• internal/log/fields.go — Added TraceIDKey and SpanIDKey constants
• internal/workflowstate/workflowstate.go — Added UpdateLoggerWithSpan method
• workflow/executor/executor.go — Call UpdateLoggerWithSpan after workflow span creation