Add DD_SITE env var to Helm deployment template (#96)

## What was changed
Added missing DD_SITE env var to the promql-to-dd-go Helm template

## Why?
Metrics are being sent to the default Datadog site but our org is on a
non-default site

1. Closes #95 

2. How was this tested:
I deployed the resulting chart using ArgoCD and the metrics are properly
forwarded to the configured DD site

---------

Co-authored-by: Alex Stanfield <[email protected]>

Author: jyanadi

cloud/observability/promql-to-dd-go/helm-charts/templates/deployment.yaml

@@ -36,6 +36,10 @@ spec:
             secretKeyRef:
               name: {{ template "promql-to-dd-go.name" . }}-secrets
               key: dd_api_key
+        {{- with .Values.dd_site }}
+        - name: DD_SITE
+          value: {{ . }}
+        {{- end }}
         volumeMounts:
         - name: secrets
           mountPath: /var/run/secrets

cloud/observability/promql-to-dd-go/helm-charts/values.yaml

@@ -7,3 +7,4 @@ image:
   tag: latest
   imagePullPolicy: Always
 query_interval_seconds: 15
+dd_site: ""