Add ability to do variable substitution

- In prometheus proxy api, allow variables to be replaced. For example,
if 'up{env="%{ci_environment_slug}"}' is passed to the endpoint, it
becomes 'up{env="production"}' before being sent to prometheus.

app/controllers/projects/environments/prometheus_api_controller.rb

@@ -7,9 +7,9 @@ class Projects::Environments::PrometheusApiController < Projects::ApplicationCon
   def proxy
     result = Prometheus::ProxyService.new(
       environment,
-      request.method,
-      params[:proxy_path],
-      params.permit!
+      proxy_method,
+      proxy_path,
+      proxy_params
     ).execute
 
     if result.nil?
@@ -31,7 +31,30 @@ class Projects::Environments::PrometheusApiController < Projects::ApplicationCon
 
   private
 
+  def query_context
+    Gitlab::Prometheus::QueryVariables.call(environment)
+  end
+
   def environment
     @environment ||= project.environments.find(params[:id])
   end
+
+  def proxy_method
+    request.method
+  end
+
+  def proxy_path
+    params[:proxy_path]
+  end
+
+  def proxy_params
+    substitute_query_variables(params).permit!
+  end
+
+  def substitute_query_variables(params)
+    query = params[:query]
+    return params unless query
+
+    params.merge(query: query % query_context)
+  end
 end

spec/controllers/projects/environments/prometheus_api_controller_spec.rb

@@ -14,6 +14,7 @@ describe Projects::Environments::PrometheusApiController do
 
   describe 'GET #proxy' do
     let(:prometheus_proxy_service) { instance_double(Prometheus::ProxyService) }
+
     let(:expected_params) do
       ActionController::Parameters.new(
         environment_params(
@@ -47,6 +48,38 @@ describe Projects::Environments::PrometheusApiController do
           expect(response).to have_gitlab_http_status(:ok)
           expect(json_response).to eq(prometheus_json_body)
         end
+
+        context 'with format string' do
+          before do
+            expected_params[:query] = %{up{environment="#{environment.slug}"}}
+          end
+
+          it 'replaces variables with values' do
+            get :proxy, params: environment_params.merge(query: 'up{environment="%{ci_environment_slug}"}')
+
+            expect(Prometheus::ProxyService).to have_received(:new)
+              .with(environment, 'GET', 'query', expected_params)
+          end
+
+          context 'with nil query' do
+            let(:params_without_query) do
+              params = environment_params
+              params.delete(:query)
+              params
+            end
+
+            before do
+              expected_params.delete(:query)
+            end
+
+            it 'does not raise error' do
+              get :proxy, params: params_without_query
+
+              expect(Prometheus::ProxyService).to have_received(:new)
+                .with(environment, 'GET', 'query', expected_params)
+            end
+          end
+        end
       end
 
       context 'with nil result' do