Add Snapshot Lifecycle metrics

[ashangit]

[lukas-vlcek]

Nicolas, do you think it would be possible to add some tests here? https://github.com/vvanholl/elasticsearch-prometheus-exporter/tree/master/src/test/resources/rest-api-spec/test/resthandler

[ashangit]

@lukas-vlcek I'm currently looking at adding some integration rest tests but to do so I need to rely on includeXpack '*' on restResources of the gradle.build But it seems that usage of includeXpack is forbidden outside of the elasticsearch build: https://github.com/elastic/elasticsearch/blob/6f02a9b0889d8dc57707842d85629b0a71b6b8d3/buildSrc/src/main/java/org/elasticsearch/gradle/test/rest/RestResourcesExtension.java#L63 This check rely on the capability of loading /buildSrc.marker from the classpath in https://github.com/elastic/elasticsearch/blob/6f02a9b0889d8dc57707842d85629b0a71b6b8d3/buildSrc/src/main/java/org/elasticsearch/gradle/info/GlobalBuildInfoPlugin.java#L95 I can try to find a way to have this marker file available so we can load those Xpack in the ES test clusters but I'm wondering if it is appropriate to do so.