Gate the prometheus exporter behind a feature

Also fixes the Docker image building and the CI builds

.github/workflows/ci.yaml

@@ -170,6 +170,9 @@ jobs:
           profile: minimal
           override: true
 
+      - name: Install Protoc
+        uses: arduino/setup-protoc@v1
+
       - name: Setup OPA
         uses: open-policy-agent/setup-opa@v1
         with:
@@ -248,6 +251,9 @@ jobs:
           profile: minimal
           override: true
 
+      - name: Install Protoc
+        uses: arduino/setup-protoc@v1
+
       - name: Setup OPA
         uses: open-policy-agent/setup-opa@v1
         with:
@@ -329,6 +335,9 @@ jobs:
           override: true
           components: llvm-tools-preview
 
+      - name: Install Protoc
+        uses: arduino/setup-protoc@v1
+
       - name: Setup OPA
         uses: open-policy-agent/setup-opa@v1
         with: