chore: added extra variables to each kokoro config (#598)

Author: daniel-sanche

.kokoro/performance/common.cfg

@@ -25,3 +25,18 @@ env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"
     value: "github/python-logging/.kokoro/test-performance.sh"
 }
+
+# add labels to help with testgrid filtering
+env_vars: {
+    key: "PRODUCT_AREA_LABEL"
+    value: "observability"
+}
+env_vars: {
+    key: "PRODUCT_LABEL"
+    value: "logging"
+}
+env_vars: {
+    key: "LANGUAGE_LABEL"
+    value: "python"
+}
+