revise Datadog trace sampling configuration (#10151)

* datadog: sample_rate omitted by default

* config: use *float32 with nil instead of float32 with sentinel value

* change some names

* gofmt -s -w internal/ingress/controller/nginx.go

internal/ingress/controller/config/config.go

@@ -706,16 +706,9 @@ type Configuration struct {
 	// Default: nginx.handle
 	DatadogOperationNameOverride string `json:"datadog-operation-name-override"`
 
-	// DatadogPrioritySampling specifies to use client-side sampling
-	// If true disables client-side sampling (thus ignoring sample_rate) and enables distributed
-	// priority sampling, where traces are sampled based on a combination of user-assigned
-	// Default: true
-	DatadogPrioritySampling bool `json:"datadog-priority-sampling"`
-
 	// DatadogSampleRate specifies sample rate for any traces created.
-	// This is effective only when datadog-priority-sampling is false
-	// Default: 1.0
-	DatadogSampleRate float32 `json:"datadog-sample-rate"`
+	// Default: use a dynamic rate instead
+	DatadogSampleRate *float32 `json:"datadog-sample-rate",omitempty`
 
 	// MainSnippet adds custom configuration to the main section of the nginx configuration
 	MainSnippet string `json:"main-snippet"`
@@ -1001,8 +994,7 @@ func NewDefault() Configuration {
 		DatadogEnvironment:                     "prod",
 		DatadogCollectorPort:                   8126,
 		DatadogOperationNameOverride:           "nginx.handle",
-		DatadogSampleRate:                      1.0,
-		DatadogPrioritySampling:                true,
+		DatadogSampleRate:                      nil,
 		LimitReqStatusCode:                     503,
 		LimitConnStatusCode:                    503,
 		SyslogPort:                             514,