Return the patched promise, not the original one

Fixes googleapis#1319

src/plugins/plugin-pg.ts

@@ -288,7 +288,7 @@ const plugin: Plugin = [
               // Unlike in pg 6, the returned value can't be both a Promise and
               // a Submittable. So we don't run the risk of double-patching
               // here.
-              pgPatch.patchPromise(pgQuery, span);
+              pgQuery = pgPatch.patchPromise(pgQuery, span);
             }
           }
           return pgQuery;