Add optional _meta object to ingest pipeline response (#2348)

Author: JoshMock

specification/ingest/_types/Pipeline.ts

@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { VersionNumber } from '@_types/common'
+import { VersionNumber, Metadata } from '@_types/common'
 import { ProcessorContainer } from './Processors'
 
 export class Pipeline {
@@ -38,6 +38,10 @@ export class Pipeline {
    * Version number used by external systems to track ingest pipelines.
    */
   version?: VersionNumber
+  /**
+   * Arbitrary metadata about the ingest pipeline. This map is not automatically generated by Elasticsearch.
+   */
+  _meta: Metadata
 }
 
 // Unused .. but let's keep it for now