Add metrics to assist monitoring and alerting (#1506)

Adds metrics to expose:
 * height of best known block
 * synchronizer in sync flag
 * max peers
 * Timestamp of current chain head
Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

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

@@ -63,18 +63,24 @@ public class DefaultMutableBlockchain implements MutableBlockchain {
     chainHeader = blockchainStorage.getBlockHeader(chainHead).get();
     totalDifficulty = blockchainStorage.getTotalDifficulty(chainHead).get();
 
-    metricsSystem.createGauge(
+    metricsSystem.createLongGauge(
         MetricCategory.BLOCKCHAIN,
         "height",
         "Height of the chainhead",
-        () -> (double) this.getChainHeadBlockNumber());
+        this::getChainHeadBlockNumber);
-    metricsSystem.createGauge(
+    metricsSystem.createLongGauge(
         MetricCategory.BLOCKCHAIN,
         "difficulty_total",
         "Total difficulty of the chainhead",
         () ->
             BytesValues.asUnsignedBigInteger(this.getChainHead().getTotalDifficulty().getBytes())
-                .doubleValue());
+                .longValue());
+
+    metricsSystem.createLongGauge(
+        MetricCategory.BLOCKCHAIN,
+        "chain_head_timestamp",
+        "Timestamp from the current chain head",
+        () -> getChainHeadHeader().getTimestamp());
   }
 
   @Override

ethereum/eth/src/main/java/tech/pegasys/pantheon/ethereum/eth/sync/DefaultSynchronizer.java

@@ -27,6 +27,7 @@ import tech.pegasys.pantheon.ethereum.eth.sync.state.PendingBlocks;
 import tech.pegasys.pantheon.ethereum.eth.sync.state.SyncState;
 import tech.pegasys.pantheon.ethereum.mainnet.ProtocolSchedule;
 import tech.pegasys.pantheon.ethereum.worldstate.WorldStateStorage;
+import tech.pegasys.pantheon.metrics.MetricCategory;
 import tech.pegasys.pantheon.metrics.MetricsSystem;
 import tech.pegasys.pantheon.util.ExceptionUtils;
 import tech.pegasys.pantheon.util.Subscribers;
@@ -95,6 +96,17 @@ public class DefaultSynchronizer<C> implements Synchronizer {
             worldStateStorage,
             syncState,
             clock);
+
+    metricsSystem.createLongGauge(
+        MetricCategory.SYNCHRONIZER,
+        "best_known_block",
+        "Height of best known block from any connected peer",
+        () -> syncState.syncStatus().getHighestBlock());
+    metricsSystem.createIntegerGauge(
+        MetricCategory.SYNCHRONIZER,
+        "in_sync",
+        "Whether or not the local node has caught up to the best known peer",
+        () -> getSyncStatus().isPresent() ? 0 : 1);
   }
 
   private TrailingPeerRequirements calculateTrailingPeerRequirements() {

ethereum/p2p/src/main/java/tech/pegasys/pantheon/ethereum/p2p/network/DefaultP2PNetwork.java

@@ -237,6 +237,12 @@ public class DefaultP2PNetwork implements P2PNetwork {
         "netty_boss_pending_tasks",
         "The number of pending tasks in the Netty boss event loop",
         pendingTaskCounter(boss));
+
+    metricsSystem.createIntegerGauge(
+        MetricCategory.NETWORK,
+        "peers_limit",
+        "Maximum P2P peer connections that can be established",
+        () -> maxPeers);
   }
 
   public static Builder builder() {