Cache-Control on Prometheus endpoint

This change adds a `Cache-Control` header on the endpoint so that reverse proxies and load balancers in front of Jenkins do not cache metrics.

src/main/java/org/jenkinsci/plugins/prometheus/MetricsRequest.java

@@ -17,6 +17,8 @@ public static HttpResponse prometheusResponse(final CollectorRegistry collectorR
             public void generateResponse(StaplerRequest request, StaplerResponse response, Object node) throws IOException, ServletException {
                 response.setStatus(StaplerResponse.SC_OK);
                 response.setContentType(TextFormat.CONTENT_TYPE_004);
+                response.addHeader("Cache-Control","must-revalidate,no-cache,no-store");
+
 
                 StringWriter buffer = new StringWriter();
                 TextFormat.write004(buffer, collectorRegistry.metricFamilySamples());
@@ -25,4 +27,4 @@ public void generateResponse(StaplerRequest request, StaplerResponse response, O
             }
         };
     }
-}
+}