mapper-exponential-histogram: add new plugin

Add a new plugin which defines the exponential_histogram
mapper type. We will later add a new aggregation for
aggregating into exponential histograms.

x-pack/plugin/mapper-exponential-histogram/build.gradle

@@ -0,0 +1,24 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+apply plugin: 'elasticsearch.internal-es-plugin'
+
+esplugin {
+  name 'exponential-histogram'
+  description 'Module for the exponential-histogram field type.'
+  classname 'org.elasticsearch.xpack.exponentialhistogram.ExponentialHistogramMapperPlugin'
+  extendedPlugins = ['x-pack-core', 'analytics']
+}
+base {
+  archivesName = 'x-pack-exponential-histogram'
+}
+
+dependencies {
+  compileOnly project(path: xpackModule('core'))
+  compileOnly project(path: xpackModule('analytics'))
+  //testImplementation project(path: ':x-pack:plugin:analytics')
+}