[HWKMETRICS-613] Use onErrorResumeNext to ignore deletion errors. If …

…a delete operation fails, it will be tried again when the expiration index is reprocessed again.

core/metrics-core-service/src/main/java/org/hawkular/metrics/core/jobs/DeleteExpiredMetrics.java

@@ -112,8 +112,9 @@ public Completable call(JobDetails jobDetails) {
 
         return expirationIndexResults
                 .concatMap(metricId -> metricsService.deleteMetric(metricId))
-                .doOnError(e -> {
+                .onErrorResumeNext(e -> {
                     logger.error("Failed to delete metric data", e);
+                    return Observable.empty();
                 })
                 .toCompletable();
     }