fix: Add 'v' prefix to default image tag for exporter

Updated the logic in `charts/iperf3-monitor/templates/exporter-controller.yaml`
to ensure that when the exporter's image tag is not specified in
`values.yaml`, it defaults to `v<Chart.AppVersion>` instead of just
`<Chart.AppVersion>`.

This change ensures the default tag matches image tagging conventions
where a 'v' prefix is used for versions (e.g., `v0.1.0`).
If an image tag is explicitly provided in `values.yaml`, that tag is
used directly without modification.

Verified with `helm template` for both default and custom tag scenarios.

charts/iperf3-monitor/templates/exporter-controller.yaml

@@ -77,7 +77,7 @@ Proceed with modifications only if the exporter controller is defined.
   {{- if $exporterContainerCfg -}}
     {{- if not $exporterContainerCfg.image.tag -}}
       {{- if $chart.AppVersion -}}
-        {{- $_ := set $exporterContainerCfg.image "tag" $chart.AppVersion -}}
+        {{- $_ := set $exporterContainerCfg.image "tag" (printf "v%s" $chart.AppVersion) -}}
       {{- else -}}
         {{- fail (printf "Error: Container image tag is not specified for controller '%s', container '%s', and Chart.AppVersion is also empty." $exporterControllerKey "exporter") -}}
       {{- end -}}