lib/promscrape: add -promscrape.cluster.memberURLTemplate command-line flag for creating direct links to vmagent instances at /service-discovery page

See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4018#issuecomment-1843811569

docs/vmagent.md

@@ -813,9 +813,15 @@ start a cluster of three `vmagent` instances, where each target is scraped by tw
 Every `vmagent` in the cluster exposes all the discovered targets at `http://vmagent:8429/service-discovery` page.
 Each discovered target on this page contains its status (`UP`, `DOWN` or `DROPPED` with the reason why the target has been dropped).
 If the target is dropped because of sharding to other `vmagent` instances in the cluster, then the status column contains
-`-promscrape.cluster.memberNum` values for `vmagent` instances where the given target is scraped. Note that `vmagent` shows
-up to `-promscrape.maxDroppedTargets` dropped targets on the `/service-discovery` page. Increase the `-promscrape.maxDroppedTargets` command-line flag value
-if the `/service-discovery` page misses some dropped targets.
+`-promscrape.cluster.memberNum` values for `vmagent` instances where the given target is scraped.
+
+The `/service-discovery` page provides links to the corresponding `vmagent` instances if `-promscrape.cluster.memberURLTemplate` command-line flag is set.
+Every occurrence of `%d` inside the `-promscrape.cluster.memberURLTemplate` is substituted with the `-promscrape.cluster.memberNum`
+for the corresponding `vmagent` instance. For examle, `-promscrape.cluster.memberURLTemplate='http://vmagent-instance-%d:8429/targets'`
+generates `http://vmagent-instance-42:8429/targets` url for `vmagent` instance, which runs with `-promscrape.cluster.memberNum=42`.
+
+Note that `vmagent` shows up to `-promscrape.maxDroppedTargets` dropped targets on the `/service-discovery` page.
+Increase the `-promscrape.maxDroppedTargets` command-line flag value if the `/service-discovery` page misses some dropped targets.
 
 If each target is scraped by multiple `vmagent` instances, then data deduplication must be enabled at remote storage pointed by `-remoteWrite.url`.
 The `-dedup.minScrapeInterval` must be set to the `scrape_interval` configured at `-promscrape.config`.

lib/promscrape/config.go

@@ -64,6 +64,10 @@ var (
 		"See also -promscrape.cluster.memberLabel . See https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets for more info")
 	clusterMemberLabel = flag.String("promscrape.cluster.memberLabel", "", "If non-empty, then the label with this name and the -promscrape.cluster.memberNum value "+
 		"is added to all the scraped metrics. See https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets for more info")
+	clusterMemberURLTemplate = flag.String("promscrape.cluster.memberURLTemplate", "", "An optional template for URL to access vmagent instance with the given -promscrape.cluster.memberNum value. "+
+		"Every %d occurence in the template is substituted with -promscrape.cluster.memberNum at urls to vmagent instances responsible for scraping the given target "+
+		"at /service-discovery page. For example -promscrape.cluster.memberURLTemplate='http://vmagent-%d:8429/targets'. "+
+		"See https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets for more details")
 	clusterReplicationFactor = flag.Int("promscrape.cluster.replicationFactor", 1, "The number of members in the cluster, which scrape the same targets. "+
 		"If the replication factor is greater than 1, then the deduplication must be enabled at remote storage side. "+
 		"See https://docs.victoriametrics.com/vmagent.html#scraping-big-number-of-targets for more info")

lib/promscrape/targetstatus.qtpl

@@ -1,5 +1,7 @@
 {% import (
 	"net/url"
+	"strconv"
+	"strings"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/htmlcomponents"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/promutils"
@@ -335,7 +337,11 @@
                                 <span title="The target exists at vmagent instances with the given -promscrape.cluster.memberNum values">
                                 exists at{% space %}
                                 {% for i, memberNum := range t.clusterMemberNums %}
-                                    {%d memberNum %}
+                                    {% if *clusterMemberURLTemplate == "" %}
+                                        {%d memberNum %}
+                                    {% else %}
+                                        <a href="{%s strings.ReplaceAll(*clusterMemberURLTemplate, "%d", strconv.Itoa(memberNum)) %}">{%d memberNum %}</a>
+                                    {% endif %}
                                     {% if i+1 < len(t.clusterMemberNums) %},{% space %}{% endif %}
                                 {% endfor %}
                             {% endif %}