Add datadogApmEnabled setting (#37)

README.md

@@ -44,6 +44,18 @@ To use a specific Datadog Java APM Agent version, add the following to your `bui
 datadogApmVersion := "0.98.1"
 ```
 
+#### `datadogApmEnabled`
+
+You can easily disable the Datadog APM by settings this setting to `false`.
+Note that if you have the `DD_TRACE_ENABLED` environment variable set, it'll take precedence over this setting.
+See https://docs.datadoghq.com/tracing/trace_collection/library_config/java/
+
+Default value: `true`
+
+```scala
+datadogApmEnabled := true
+```
+
 #### `datadogServiceName`
 
 By default, the agent will use the sbt project `name` value as `service.name`. 

src/main/scala/com/guizmaii/sbt/DatadogAPM.scala

@@ -31,6 +31,10 @@ object DatadogAPM extends AutoPlugin {
 
     lazy val datadogJavaAgent = taskKey[File]("Datadog agent jar location")
 
+    lazy val datadogApmEnabled = taskKey[Boolean](
+      "Datadog APM agent enabled. Default: `DD_TRACE_ENABLED` envvar value if present, 'true' otherwise"
+    )
+
     lazy val datadogServiceName = taskKey[String](
       "The name of a set of processes that do the same job. Used for grouping stats for your application. Default value is the sbt project name"
     )
@@ -56,13 +60,15 @@ object DatadogAPM extends AutoPlugin {
     ivyConfigurations += DatadogConfig,
     datadogApmVersion                              := "0.100.0",
     datadogJavaAgent                               := findDatadogJavaAgent(update.value),
+    datadogApmEnabled                              := true,
     datadogServiceName                             := name.value,
     datadogAgentTraceUrl                           := TraceAgentUrl.defaultUnixSocketUrl,
     datadogEnableDebug                             := false,
     datadogGlobalTags                              := Map.empty,
     libraryDependencies += "com.datadoghq"          % "dd-java-agent" % datadogApmVersion.value % DatadogConfig,
     Universal / mappings += datadogJavaAgent.value -> "datadog/dd-java-agent.jar",
     bashScriptExtraDefines += """addJava "-javaagent:${app_home}/../datadog/dd-java-agent.jar"""",
+    bashScriptExtraDefines += s"""addJava "-Ddd.trace.enabled=${datadogApmEnabled.value}"""",
     bashScriptExtraDefines += s"""addJava "-Ddd.service.name=${datadogServiceName.value}"""",
     bashScriptExtraDefines += {
       datadogAgentTraceUrl.value match {