Prevent integration failure when graph-analytics-plugin is not applied (

#38)

Ensures that graph-analytics-plugin code references are only in play when
the plugin has been applied.

Updates the docs to clarify the requirement to separately apply the other
plugin.

src/main/kotlin/com/ebay/plugins/metrics/develocity/projectcost/ProjectCostPlugin.kt

@@ -83,15 +83,15 @@ internal class ProjectCostPlugin @Inject constructor(
 
         val projectCostGraphAnalysis = providerFactory.gradleProperty(GRAPH_ANALYSIS_ENABLED_PROPERTY).orNull.toBoolean()
         if (projectCostGraphAnalysis) {
-            val reportDuration = providerFactory.gradleProperty(GRAPH_ANALYSIS_DURATION_PROPERTY).orElse("P7D").get()
-            val reportTaskProvider = reportTaskProviderFun.invoke(reportDuration)
-            val graphAnalysisTask = project.tasks.register("projectCostGraphAnalysis-$reportDuration", ProjectCostGraphAnalysisTask::class.java)
-            graphAnalysisTask.configure { task ->
-                with(task) {
-                    projectReportProperty.set(reportTaskProvider.flatMap { it.reportFile })
-                }
-            }
             project.plugins.withId("com.ebay.graph-analytics") {
+                val reportDuration = providerFactory.gradleProperty(GRAPH_ANALYSIS_DURATION_PROPERTY).orElse("P7D").get()
+                val reportTaskProvider = reportTaskProviderFun.invoke(reportDuration)
+                val graphAnalysisTask = project.tasks.register("projectCostGraphAnalysis-$reportDuration", ProjectCostGraphAnalysisTask::class.java)
+                graphAnalysisTask.configure { task ->
+                    with(task) {
+                        projectReportProperty.set(reportTaskProvider.flatMap { it.reportFile })
+                    }
+                }
                 project.extensions.getByType(GraphExtension::class.java).apply {
                     analysisTasks.add(graphAnalysisTask)
                 }

src/main/kotlin/com/ebay/plugins/metrics/develocity/projectcost/README.md

@@ -35,15 +35,18 @@ performed by the [eBay Graph Analytics Plugin](https://github.com/eBay/graph-ana
 This integration allows for the resulting project graph data to include the execution-time
 costs for each project module, granting a more comprehensive picture of the project's build.
 
-To enable the integration, the gradle property `projectCostGraphAnalysisEnabled` must be
-set to `true`.  Since the data gathering phase for the project cost summarizer can be quite
+To enable the integration, the following must be done:
+- The gradle property `projectCostGraphAnalysisEnabled` must be set to `true`.  This activates the code
+  which performs the integration work.
+- The `graph-analytics-plugin` must be (separately) applied to the project.  This makes the implementation
+  available to the project, allowing the integration to be performed.
+- The `projectCostGraphAnalysisDuration` property may be set to a valid Java time duration string.
+  This specifies the duration of time over which the project cost data will be aggregated.  If not
+  specified, the default value of `P7D` (7 days) will be used.
+
+Since the data gathering phase for the project cost summarizer can be quite
 expensive to gather, it is recommended to run this integration in a dedicated job.
 
-Additionally, since the project cost reporting aggregates data over a period of time, the
-duration for the report must be specified.  This is done by setting the
-`projectCostGraphAnalysisDuration` property to a Java time duration string.  If not specified,
-data form the last 7 days will be used.
-
 Putting it all together, the following command can be used to generate a graph analysis
 incorporating the project cost data over a custom time range of 3 days: