Docs for Eventing upgrade tests config overrides.

test/upgrade/README.md

@@ -83,3 +83,21 @@ Diagram below describe the setup:
           +--------+       +---------+ |
            (default)        +----------+
 ```
+
+#### Probe test configuration
+
+Probe test behavior can be influenced from outside without modifying its source
+code. That can be beneficial if one would like to run upgrade tests in different
+context. One such example might be running Eventing upgrade tests in place that
+have Serving and Eventing both installed. In such environment one can set
+environment variable `E2E_UPGRADE_TESTS_SERVING_USE` to enable usage of ksvc
+forwarder (which is disabled by default):
+
+```
+$ export E2E_UPGRADE_TESTS_SERVING_USE=true
+```
+
+Any option, apart from namespace, in
+[`knative.dev/eventing/test/upgrade/prober.Config`](https://github.com/knative/eventing/blob/022e281/test/upgrade/prober/prober.go#L52-L63)
+struct can be influenced, by using `E2E_UPGRADE_TESTS_XXXXX` environmental
+variable prefix (using [kelseyhightower/envconfig] usage).

test/upgrade/prober/prober.go

@@ -64,19 +64,20 @@ type ServingConfig struct {
 }
 
 func NewConfig(namespace string) *Config {
-	servingConfig := ServingConfig{
-		Use:         false,
-		ScaleToZero: true,
-	}
-	err := envconfig.Process("e2e_upgrade_tests_serving", &servingConfig)
-	ensure.NoError(err)
-	return &Config{
-		Namespace:     namespace,
+	config := &Config{
+		Namespace:     "",
 		Interval:      Interval,
 		FinishedSleep: 5 * time.Second,
 		FailOnErrors:  true,
-		Serving:       servingConfig,
+		Serving:       ServingConfig{
+			Use:         false,
+			ScaleToZero: true,
+		},
 	}
+	err := envconfig.Process("e2e_upgrade_tests", config)
+	ensure.NoError(err)
+	config.Namespace = namespace
+	return config
 }
 
 // RunEventProber starts a single Prober of the given domain.