Get `_node/pipelines?graph=true` output for Logstash to allow tracing metrics to plugins

As noted in https://github.com/elastic/logstash/issues/12228 , the `_node/pipelines?graph=true` output is useful for tracing metrics back to the particular plugins they are associated with. See the referenced issue for an example. It would be helpful to retrieve this output as part of the logstash diagnostics.