HBASE-27395 Adding description to Prometheus metrics (#4807)

Signed-off-by: Tamas Payer <payert@apache.org>
Signed-off-by: Balazs Meszaros <meszibalu@apache.org>

hbase-http/src/main/java/org/apache/hadoop/hbase/http/prometheus/PrometheusHadoopServlet.java

@@ -41,7 +41,7 @@ public class PrometheusHadoopServlet extends HttpServlet {
 
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
-    writeMetrics(resp.getWriter());
+    writeMetrics(resp.getWriter(), "true".equals(req.getParameter("description")));
   }
 
   static String toPrometheusName(String metricRecordName, String metricName) {
@@ -57,15 +57,21 @@ static String toPrometheusName(String metricRecordName, String metricName) {
    */
   @RestrictedApi(explanation = "Should only be called in tests or self", link = "",
       allowedOnPath = ".*/src/test/.*|.*/PrometheusHadoopServlet\\.java")
-  void writeMetrics(Writer writer) throws IOException {
+  void writeMetrics(Writer writer, boolean desc) throws IOException {
     Collection<MetricsRecord> metricRecords = MetricsExportHelper.export();
     for (MetricsRecord metricsRecord : metricRecords) {
       for (AbstractMetric metrics : metricsRecord.metrics()) {
         if (metrics.type() == MetricType.COUNTER || metrics.type() == MetricType.GAUGE) {
 
           String key = toPrometheusName(metricsRecord.name(), metrics.name());
+
+          if (desc) {
+            String description = metrics.description();
+            if (!description.isEmpty()) writer.append("# HELP ").append(description).append('\n');
+          }
+
           writer.append("# TYPE ").append(key).append(" ")
-            .append(metrics.type().toString().toLowerCase()).append("\n").append(key).append("{");
+            .append(metrics.type().toString().toLowerCase()).append('\n').append(key).append("{");
 
           /* add tags */
           String sep = "";

hbase-http/src/test/java/org/apache/hadoop/hbase/http/prometheus/TestPrometheusServlet.java

@@ -60,7 +60,8 @@ public void testPublish() throws IOException {
 
     // WHEN
     PrometheusHadoopServlet prom2Servlet = new PrometheusHadoopServlet();
-    prom2Servlet.writeMetrics(writer);
+    // Test with no description
+    prom2Servlet.writeMetrics(writer, false);
 
     // THEN
     String writtenMetrics = stream.toString(UTF_8.name());