Fix assert in grpc_timer_init(). https://github.com/grpc/grpc/issues/11512

src/core/lib/security/credentials/oauth2/oauth2_credentials.c

@@ -296,10 +296,10 @@ static bool oauth2_token_fetcher_get_request_metadata(
   gpr_mu_unlock(&c->mu);
   if (start_fetch) {
     grpc_call_credentials_ref(creds);
-    c->fetch_func(exec_ctx, grpc_credentials_metadata_request_create(creds),
-                  &c->httpcli_context, &c->pollent,
-                  on_oauth2_token_fetcher_http_response,
-                  gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
+    c->fetch_func(
+        exec_ctx, grpc_credentials_metadata_request_create(creds),
+        &c->httpcli_context, &c->pollent, on_oauth2_token_fetcher_http_response,
+        gpr_time_add(gpr_now(GPR_CLOCK_MONOTONIC), refresh_threshold));
   }
   return false;
 }