Improve log info and condition about fetching metrics (#219)

- Receiving "No metrics" response won't cause error log
- Improve log about fetching metrics

sentinel-dashboard/src/main/java/com/taobao/csp/sentinel/dashboard/discovery/MachineInfo.java

@@ -130,4 +130,13 @@ public class MachineInfo implements Comparable<MachineInfo> {
     public int hashCode() {
         return Objects.hash(app, ip, port);
     }
+
+    /**
+     * Information for log
+     *
+     * @return
+     */
+    public String toLogString() {
+        return app + "|" + ip + "|" + port + "|" + version;
+    }
 }

sentinel-dashboard/src/main/java/com/taobao/csp/sentinel/dashboard/metric/MetricFetcher.java

@@ -64,15 +64,14 @@ import org.springframework.stereotype.Component;
 @Component
 public class MetricFetcher {
 
-    private static Logger logger = LoggerFactory.getLogger(MetricFetcher.class);
-    private static final int HTTP_OK = 200;
-
     public static final long MAX_CLIENT_LIVE_TIME_MS = 1000 * 60 * 5;
+    public static final String NO_METRICS = "No metrics";
+    private static final int HTTP_OK = 200;
     private static final long MAX_LAST_FETCH_INTERVAL_MS = 1000 * 15;
     private static final long FETCH_INTERVAL_SECOND = 6;
     private static final Charset DEFAULT_CHARSET = Charset.forName(SentinelConfig.charset());
     private final static String METRIC_URL_PATH = "metric";
-
+    private static Logger logger = LoggerFactory.getLogger(MetricFetcher.class);
     private final long intervalSecond = 1;
 
     private Map<String, AtomicLong> appLastFetchTime = new ConcurrentHashMap<>();
@@ -279,7 +278,7 @@ public class MetricFetcher {
         } catch (Exception ignore) {
         }
         String body = EntityUtils.toString(response.getEntity(), charset != null ? charset : DEFAULT_CHARSET);
-        if (StringUtil.isEmpty(body)) {
+        if (StringUtil.isEmpty(body) || body.startsWith(NO_METRICS)) {
             //logger.info(machine.getApp() + ":" + machine.getIp() + ":" + machine.getPort() + ", bodyStr is empty");
             return;
         }
@@ -322,7 +321,7 @@ public class MetricFetcher {
                     map.put(key, entity);
                 }
             } catch (Exception e) {
-                logger.info("handleBody line error: {}", line);
+                logger.warn("handleBody line exception, machine: {}, line: {}", machine.toLogString(), line);
             }
         }
     }