Isolate polling of pollables in StatsdMeterRegistry (#2549)

This prevents a misbehaving pollable meter (a gauge with a custom value function that might throw an NPE, for example) from preventing other ongoing polling of other pollable meters.

Fixes gh-2543

implementations/micrometer-registry-statsd/src/main/java/io/micrometer/statsd/StatsdMeterRegistry.java

@@ -161,7 +161,11 @@ private <M extends Meter> void removePollableMeter(M m) {
 
  void poll() {
  for (StatsdPollable pollableMeter : pollableMeters.values()) {
- pollableMeter.poll();
+ try {
+ pollableMeter.poll();
+ } catch (RuntimeException e) {
+ // Silently ignore misbehaving pollable meter
+ }
  }
  }