fix: set the value of elasticsearch_clusterinfo_version_info to 1 (prometheus-community#728)

In the Prometheus ecosystem, info metrics (those with the `_info`
suffix) usually have a value of 1. Although not enforced, this has
become a standard pattern across Prometheus and several exporters (see
`prometheus_build_info`).

Signed-off-by: Jorge Luis Betancourt Gonzalez <[email protected]>

Author: jorgelbg

pkg/clusterinfo/clusterinfo.go

@@ -146,7 +146,7 @@ func (r *Retriever) updateMetrics(res *Response) {
 		res.Version.BuildHash,
 		res.Version.Number.String(),
 		res.Version.LuceneVersion.String(),
-	)
+	).Set(1.0)
 	r.lastUpstreamSuccessTs.WithLabelValues(url).Set(float64(time.Now().Unix()))
 }