Fix JSON tag mismatch for metrics responses

[Nehanth67]

CloudStack metrics APIs return lists under updated JSON keys (e.g. "host" for host metrics).
The cloudstack-go response structs still used older json tags (e.g. "hostmetric"/"hostmetrics"),
causing unmarshalling issues where metrics lists become nil/null.
This PR updates the relevant response struct json tags to match the actual CloudStack API response keys.

[Copilot]

Pull request overview

This PR fixes JSON unmarshalling issues in CloudStack metrics APIs by updating response struct tags to match the actual API response keys. The CloudStack API returns metrics lists under singular entity names (e.g., "host", "zone"), but the Go structs were using incorrect plural/compound tags (e.g., "hostsmetric", "zonesmetric").

• Updated JSON tags for 6 metrics response structs to use singular entity names
• Aligned naming convention with other existing metrics responses in the codebase (ManagementServersMetrics, StoragePoolsMetrics, VirtualMachinesMetrics already use singular names)

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ZoneService.go	Changed ZonesMetrics JSON tag from "zonesmetric" to "zone"
VolumeService.go	Changed VolumesMetrics JSON tag from "volumesmetric" to "volume"
UsageService.go	Changed UsageServerMetrics JSON tag from "usageservermetric" to "usageserver"
InfrastructureUsageService.go	Changed DbMetrics JSON tag from "dbMetrics" to "db"
HostService.go	Changed HostsMetrics JSON tag from "hostsmetric" to "host"
ClusterService.go	Changed ClustersMetrics JSON tag from "clustersmetric" to "cluster"

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[DaanHoogland]

clgtm

[DaanHoogland]

@Pearl1594 @vishesh92 , please review this?

[jmsperu]

Reviewed these against the CloudStack 4.22 source (plugins/metrics/src/main/java/org/apache/cloudstack/metrics/MetricsServiceImpl.java). The host / cluster / volume / zone changes look correct (they match the object names inherited from the base responses / set in the service). Two look off, though:

• DbMetricsResponse — this changes json:"dbMetrics" → json:"db", but listDbMetrics() calls response.setObjectName("dbMetrics") (MetricsServiceImpl.java:1114). So the original dbMetrics tag was already correct, and db would actually break DB-metrics unmarshalling. Suggest reverting this one to dbMetrics.
• UsageServerMetrics — changed to json:"usageserver", but listUsageServerMetrics() calls response.setObjectName("usageMetrics") (MetricsServiceImpl.java:1087). The key is usageMetrics, not usageserver.

The other four are good to merge. Happy to confirm against a live 4.22 deployment once I have one to hand.

[jmsperu]

Confirmed now against a live CloudStack 4.22 deployment (raw response=json output):

API	actual response key
listDbMetrics	dbMetrics
listUsageServerMetrics	usageMetrics
listHostsMetrics	host
listClustersMetrics	cluster

So host / cluster (and volume / zone) are correct as in this PR. The two to fix:

• DbMetrics: keep json:"dbMetrics" (the original) — db returns nothing.
• UsageServerMetrics: use json:"usageMetrics", not usageserver.

With those two tweaked, this is good to merge. 👍