Metrics measurement adjustment (#511)

The total difficulty of mainnet is larger than a signed long, so we
need to adjust the way we get the number.

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

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/db/DefaultMutableBlockchain.java

@@ -32,6 +32,7 @@ import tech.pegasys.pantheon.ethereum.util.InvalidConfigurationException;
 import tech.pegasys.pantheon.metrics.MetricCategory;
 import tech.pegasys.pantheon.metrics.MetricsSystem;
 import tech.pegasys.pantheon.util.Subscribers;
+import tech.pegasys.pantheon.util.bytes.BytesValues;
 import tech.pegasys.pantheon.util.uint.UInt256;
 
 import java.util.ArrayList;
@@ -69,7 +70,9 @@ public class DefaultMutableBlockchain implements MutableBlockchain {
         MetricCategory.BLOCKCHAIN,
         "difficulty_total",
         "Total difficulty of the chainhead",
-        () -> (double) this.getChainHead().getTotalDifficulty().toLong());
+        () ->
+            BytesValues.asUnsignedBigInteger(this.getChainHead().getTotalDifficulty().getBytes())
+                .doubleValue());
   }
 
   @Override