sql: prevent span use after finish with rollbacks and high vmodule

Previously, we could get a panic of "span use after finish" in test
builds in some cases (like a rollback). This was occurring since the
context cancellation function can log some stuff depending on the
verbosity level (in `contextutil/context.go:74`) long time after we have
finished the tracing span from that context. To go around that issue we
now push the responsibility of finishing the span into the cancellation
function.

Release note: None

pkg/sql/exec_util.go

@@ -2285,9 +2285,17 @@ func (st *SessionTracing) StartTracing(
 		if sp == nil {
 			return errors.Errorf("no txn span for SessionTracing")
 		}
-		// We're hijacking this span and we're never going to un-hijack it, so it's
-		// up to us to finish it.
-		sp.Finish()
+		// Since we're hijacking this span and we're never going to un-hijack
+		// it, it's up to us to make sure it is finished somehow. Unfortunately,
+		// we cannot just finish it right away because in some cases the span
+		// will be used later (if the verbosity is high enough, during the
+		// context cancellation). To go around that, we pass the responsibility
+		// of finishing the span to the cancellation function.
+		cancelTxnCtx := st.ex.state.cancel
+		st.ex.state.cancel = func() {
+			cancelTxnCtx()
+			sp.Finish()
+		}
 
 		st.ex.state.Ctx, _ = tracing.EnsureChildSpan(
 			newConnCtx, st.ex.server.cfg.AmbientCtx.Tracer, "session tracing")