Fix spelling error in MetricCategory.getApplicationPrefix method name (#1711)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

metrics/core/src/main/java/tech/pegasys/pantheon/metrics/MetricCategory.java

@@ -18,5 +18,5 @@ public interface MetricCategory {
 
   String getName();
 
-  Optional<String> getAppliationPrefix();
+  Optional<String> getApplicationPrefix();
 }

metrics/core/src/main/java/tech/pegasys/pantheon/metrics/PantheonMetricCategory.java

@@ -64,7 +64,7 @@ public enum PantheonMetricCategory implements MetricCategory {
   }
 
   @Override
-  public Optional<String> getAppliationPrefix() {
+  public Optional<String> getApplicationPrefix() {
     return pantheonSpecific ? PANTHEON_PREFIX : Optional.empty();
   }
 }

metrics/core/src/main/java/tech/pegasys/pantheon/metrics/StandardMetricCategory.java

@@ -30,7 +30,7 @@ public enum StandardMetricCategory implements MetricCategory {
   }
 
   @Override
-  public Optional<String> getAppliationPrefix() {
+  public Optional<String> getApplicationPrefix() {
     return Optional.empty();
   }
 }

metrics/core/src/main/java/tech/pegasys/pantheon/metrics/prometheus/PrometheusMetricsSystem.java

@@ -241,7 +241,7 @@ public class PrometheusMetricsSystem implements MetricsSystem {
   }
 
   private String prometheusPrefix(final MetricCategory category) {
-    return category.getAppliationPrefix().orElse("") + category.getName() + "_";
+    return category.getApplicationPrefix().orElse("") + category.getName() + "_";
   }
 
   CollectorRegistry getRegistry() {