reduce warning volume for sidecar registry (Netflix#1020)

Some writers such as the UDP writer can be quite noisy if
the sidecar is not present. To avoid spamming the user with
warnings, they will be suppressed after a warning is logged.
Warnings will be re-enabled if there is a successful write
so that if there are problems with the sidecar being
intermittently accessible it will still get detected.

spectator-reg-sidecar/src/main/java/com/netflix/spectator/sidecar/SidecarWriter.java

@@ -65,8 +65,11 @@ static SidecarWriter create(String location) {
 
   private final String location;
 
+  private volatile boolean suppressWarnings;
+
   SidecarWriter(String location) {
     this.location = location;
+    this.suppressWarnings = false;
   }
 
   abstract void writeImpl(String line) throws IOException;
@@ -75,8 +78,16 @@ void write(String line) {
     try {
       LOGGER.trace("writing to {}: {}", location, line);
       writeImpl(line);
+      suppressWarnings = false;
     } catch (IOException e) {
-      LOGGER.warn("write to {} failed: {}", location, line, e);
+      // Some writers such as the UDP writer can be quite noisy if the sidecar is not present.
+      // To avoid spamming the user with warnings, they will be suppressed after a warning is
+      // logged. Warnings will be re-enabled if there is a successful write so that if there
+      // are problems with a sidecar being intermittently accessible it will still get detected.
+      if (!suppressWarnings) {
+        LOGGER.warn("write to {} failed: {}", location, line, e);
+        suppressWarnings = true;
+      }
     }
   }