feat: Expose SpanFromContext (#672)

Co-authored-by: Andrii Zhezhel <andrii.zhezhel@inflect.com>
Co-authored-by: Anton Ovchinnikov <anton@tonyo.info>

tracing.go

@@ -956,27 +956,9 @@ func TransactionFromContext(ctx context.Context) *Span {
 	return nil
 }
 
-// spanFromContext returns the last span stored in the context or a dummy
-// non-nil span.
-//
-// TODO(tracing): consider exporting this. Without this, users cannot retrieve a
-// span from a context since spanContextKey is not exported.
-//
-// This can be added retroactively, and in the meantime think better whether it
-// should return nil (like GetHubFromContext), always non-nil (like
-// HubFromContext), or both: two exported functions.
-//
-// Note the equivalence:
-//
-//	SpanFromContext(ctx).StartChild(...) === StartSpan(ctx, ...)
-//
-// So we don't aim spanFromContext at creating spans, but mutating existing
-// spans that you'd have no access otherwise (because it was created in code you
-// do not control, for example SDK auto-instrumentation).
-//
-// For now we provide TransactionFromContext, which solves the more common case
-// of setting tags, etc, on the current transaction.
-func spanFromContext(ctx context.Context) *Span {
+// SpanFromContext returns the last span stored in the context, or nil if no span
+// is set on the context.
+func SpanFromContext(ctx context.Context) *Span {
 	if span, ok := ctx.Value(spanContextKey{}).(*Span); ok {
 		return span
 	}

tracing_test.go

@@ -389,7 +389,7 @@ func (c SpanCheck) Check(t *testing.T, span *Span) {
 		t.Errorf("original context value lost")
 	}
 	// Invariant: SpanFromContext(span.Context) == span
-	if spanFromContext(gotCtx) != span {
+	if SpanFromContext(gotCtx) != span {
 		t.Errorf("span not in its context")
 	}
 
@@ -586,7 +586,7 @@ func TestSpanFromContext(t *testing.T) {
 	// SpanFromContext(ctx).StartChild(...) === StartSpan(ctx, ...)
 
 	ctx := NewTestContext(ClientOptions{})
-	span := spanFromContext(ctx)
+	span := SpanFromContext(ctx)
 
 	_ = span