lib/discovery/azure: fix host check in next link in Azure SD (#6915)

Previous bugfix at 49f63b2 only partially fixed pagination host validation error.

 Before this fix it was:
```
unexpected nextLink host \"management.azure.com\", expecting \"https://management.azure.com\"
```

Now we only check the `Host` without schema. 

However, when Azure respond `nextLink` in `Host:Port` format, the
`nextLink` check will fail:
```
unexpected nextLink host \"management.azure.com:443\", expecting \"management.azure.com\"
```

This pull request further relaxes the checks by only checking the
`Hostname`.

---

 related issue: https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6912

docs/changelog/CHANGELOG.md

@@ -23,6 +23,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): perform deduplication for all received data when specifying `-streamAggr.dedupInterval` or `-remoteWrite.streamAggr.dedupInterval` command-line flags are set. Previously, if the `-remoteWrite.streamAggr.config` or `-streamAggr.config` is set, only series that matched aggregation config were deduplicated. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6711#issuecomment-2288361213) for details.
 * FEATURE: all VictoriaMetrics [enterprise](https://docs.victoriametrics.com/enterprise/) components: add support of hot-reload for license key supplied by `-licenseFile` command-line flag.
 
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/) fix service discovery of Azure Virtual Machines for response contains `nextLink` in `Host:Port` format. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6912).
 * BUGFIX: [vmagent dashboard](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/dashboards/vmagent.json): fix legend captions for stream aggregation related panels. Before they were displaying wrong label names.
 * BUGFIX: [vmgateway](https://docs.victoriametrics.com/vmgateway/): add missing `datadog`, `newrelic`, `opentelemetry` and `pushgateway` routes to the `JWT` authorization routes. Allows prefixed (`promtheus/graphite`) routes for query requests.
 * BUGFIX: [Single-node VictoriaMetrics](https://docs.victoriametrics.com/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): fix metric `vm_object_references{type="indexdb"}`. Previously, it was overcounted.

lib/promscrape/discovery/azure/api.go

@@ -122,7 +122,7 @@ func newAPIConfig(sdc *SDConfig, baseDir string) (*apiConfig, error) {
 
 	cfg := &apiConfig{
 		c:              c,
-		apiServerHost:  u.Host,
+		apiServerHost:  u.Hostname(),
 		port:           port,
 		resourceGroup:  sdc.ResourceGroup,
 		subscriptionID: sdc.SubscriptionID,

lib/promscrape/discovery/azure/machine.go

@@ -96,8 +96,10 @@ func visitAllAPIObjects(ac *apiConfig, apiURL string, cb func(data json.RawMessa
 			return fmt.Errorf("cannot parse nextLink from response %q: %w", lar.NextLink, err)
 		}
 
-		if nextURL.Host != "" && nextURL.Host != ac.apiServerHost {
-			return fmt.Errorf("unexpected nextLink host %q, expecting %q", nextURL.Host, ac.apiServerHost)
+		// Sometimes Azure will respond a host with a port. Since all possible apiServer defined in cloudEnvironments do not include a port,
+		// it is best to check the host without the port. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6912
+		if nextURL.Host != "" && nextURL.Hostname() != ac.apiServerHost {
+			return fmt.Errorf("unexpected nextLink host %q, expecting %q", nextURL.Hostname(), ac.apiServerHost)
 		}
 
 		nextLinkURI = nextURL.RequestURI()

lib/promscrape/discovery/azure/machine_test.go

@@ -91,7 +91,7 @@ func TestGetVirtualMachinesSuccess(t *testing.T) {
 			defer c.Stop()
 			ac := &apiConfig{
 				c:              c,
-				apiServerHost:  u.Host,
+				apiServerHost:  u.Hostname(),
 				subscriptionID: "some-id",
 				refreshToken: func() (string, time.Duration, error) {
 					return "auth-token", 0, nil