fix: Updating metric registrations in the start function of the EMATh…

…roughputSampler (#845)

Updating the name of the metrics being registered during the Start
function in the EMAThroughputSampler

## Which problem is this PR solving?

- #844 

## Short description of the changes

Change Register calls from this:
```go
	d.Metrics.Register("dynsampler_num_dropped", "counter")
	d.Metrics.Register("dynsampler_num_kept", "counter")
	d.Metrics.Register("dynsampler_sample_rate", "histogram")
```

to this:
```go
	d.Metrics.Register(d.prefix+"num_dropped", "counter")
	d.Metrics.Register(d.prefix+"num_kept", "counter")
	d.Metrics.Register(d.prefix+"sample_rate", "histogram")
```

Signed-off-by: Davin Taddeo <[email protected]>
Co-authored-by: Kent Quirk <[email protected]>

sample/ema_throughput.go

@@ -74,9 +74,9 @@ func (d *EMAThroughputSampler) Start() error {
 	for name := range d.lastMetrics {
 		d.Metrics.Register(name, getMetricType(name))
 	}
-	d.Metrics.Register("dynsampler_num_dropped", "counter")
-	d.Metrics.Register("dynsampler_num_kept", "counter")
-	d.Metrics.Register("dynsampler_sample_rate", "histogram")
+	d.Metrics.Register(d.prefix+"num_dropped", "counter")
+	d.Metrics.Register(d.prefix+"num_kept", "counter")
+	d.Metrics.Register(d.prefix+"sample_rate", "histogram")
 
 	return nil
 }