MM-62365: Remove unused field from MetricSample (#30991)

The Timestamp field was not used at all. Therefore
removing it.

https://mattermost.atlassian.net/browse/MM-62365

```release-note
NONE
```

server/public/model/metrics.go

@@ -84,10 +84,9 @@ var (
 )
 
 type MetricSample struct {
-	Metric    MetricType        `json:"metric"`
-	Value     float64           `json:"value"`
-	Timestamp float64           `json:"timestamp,omitempty"`
-	Labels    map[string]string `json:"labels,omitempty"`
+	Metric MetricType        `json:"metric"`
+	Value  float64           `json:"value"`
+	Labels map[string]string `json:"labels,omitempty"`
 }
 
 func (s *MetricSample) GetLabelValue(name string, acceptedValues map[string]any, defaultValue string) string {