Avoid polluting download stats on builds (elastic#38660)

Recently we changed where we source released artifacts for usage in
backwards compatibility tests. We now source these from
artifacts.elastic.co. To avoid polluting the download stats from builds,
we want to add the X-Elastic-No-KPI header to requests from
artifacts.elastic.co. To do this, we hack the Ivy feature of custom HTTP
header credentials and specify our desired headers.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -39,6 +39,7 @@ import org.gradle.api.artifacts.ModuleVersionIdentifier
 import org.gradle.api.artifacts.ProjectDependency
 import org.gradle.api.artifacts.ResolvedArtifact
 import org.gradle.api.artifacts.dsl.RepositoryHandler
+import org.gradle.api.credentials.HttpHeaderCredentials
 import org.gradle.api.execution.TaskExecutionGraph
 import org.gradle.api.plugins.JavaBasePlugin
 import org.gradle.api.plugins.JavaPlugin
@@ -50,6 +51,7 @@ import org.gradle.api.tasks.bundling.Jar
 import org.gradle.api.tasks.compile.GroovyCompile
 import org.gradle.api.tasks.compile.JavaCompile
 import org.gradle.api.tasks.javadoc.Javadoc
+import org.gradle.authentication.http.HttpHeaderAuthentication
 import org.gradle.internal.jvm.Jvm
 import org.gradle.process.ExecResult
 import org.gradle.process.ExecSpec
@@ -570,6 +572,14 @@ class BuildPlugin implements Plugin<Project> {
             patternLayout {
                 artifact "elasticsearch/[module]-[revision](-[classifier]).[ext]"
             }
+            // this header is not a credential but we hack the capability to send this header to avoid polluting our download stats
+            credentials(HttpHeaderCredentials) {
+                name = "X-Elastic-No-KPI"
+                value = "1"
+            }
+            authentication {
+                header(HttpHeaderAuthentication)
+            }
         }
         repos.maven {
             name "elastic"