feat(metrics): add block transaction count histogram for empty block monitoring

Replace the dedicated tron:block_empty_total counter with a more comprehensive
tron:block_transaction_count histogram that tracks the distribution of
transaction counts per block.

Changes:
- Add overloaded init() method in MetricsHistogram to support custom buckets
- Add BLOCK_TRANSACTION_COUNT histogram with buckets [0, 10, 50, 100, 200, 500, 1000, 2000, 5000, 10000]
- Record transaction count for all blocks (including empty blocks with txCount=0)
- Empty blocks can be queried via bucket le=0.0
- Remove unused BLOCK_EMPTY counter
- Keep SR_SET_CHANGE counter for SR set change monitoring

This provides richer insights for network analysis while still supporting
empty block monitoring via histogram bucket queries.

Closes #6590

Author: wijnaldum-eth

common/src/main/java/org/tron/common/prometheus/MetricKeys.java

@@ -17,7 +17,6 @@ public static class Counter {
     public static final String P2P_ERROR = "tron:p2p_error";
     public static final String P2P_DISCONNECT = "tron:p2p_disconnect";
     public static final String INTERNAL_SERVICE_FAIL = "tron:internal_service_fail";
-    public static final String BLOCK_EMPTY   = "tron:block_empty_total";
     public static final String SR_SET_CHANGE = "tron:sr_set_change_total";
 
     private Counter() {
@@ -64,6 +63,7 @@ public static class Histogram {
     public static final String MESSAGE_PROCESS_LATENCY = "tron:message_process_latency_seconds";
     public static final String BLOCK_FETCH_LATENCY = "tron:block_fetch_latency_seconds";
     public static final String BLOCK_RECEIVE_DELAY = "tron:block_receive_delay_seconds";
+    public static final String BLOCK_TRANSACTION_COUNT = "tron:block_transaction_count";
 
     private Histogram() {
       throw new IllegalStateException("Histogram");

common/src/main/java/org/tron/common/prometheus/MetricsCounter.java

@@ -18,9 +18,6 @@ class MetricsCounter {
     init(MetricKeys.Counter.P2P_DISCONNECT, "tron p2p disconnect .", "type");
     init(MetricKeys.Counter.INTERNAL_SERVICE_FAIL, "internal Service fail.",
         "class", "method");
-    init(MetricKeys.Counter.BLOCK_EMPTY,
-        "Total number of empty blocks (no transactions).",
-        "type");
     init(MetricKeys.Counter.SR_SET_CHANGE,
         "Total SR set changes during maintenance periods.",
         "witness", "change_type");

common/src/main/java/org/tron/common/prometheus/MetricsHistogram.java

@@ -48,6 +48,10 @@ public class MetricsHistogram {
     init(MetricKeys.Histogram.BLOCK_FETCH_LATENCY, "fetch block latency.");
     init(MetricKeys.Histogram.BLOCK_RECEIVE_DELAY,
         "receive block delay time, receiveTime - blockTime.");
+    init(MetricKeys.Histogram.BLOCK_TRANSACTION_COUNT,
+        "Distribution of transaction counts per block.",
+        new double[]{0, 10, 50, 100, 200, 500, 1000, 2000, 5000, 10000},
+        "miner");
   }
 
   private MetricsHistogram() {
@@ -62,6 +66,17 @@ private static void init(String name, String help, String... labels) {
         .register());
   }
 
+  private static void init(String name, String help, double[] buckets, String... labels) {
+    Histogram.Builder builder = Histogram.build()
+        .name(name)
+        .help(help)
+        .labelNames(labels);
+    if (buckets != null && buckets.length > 0) {
+      builder.buckets(buckets);
+    }
+    container.put(name, builder.register());
+  }
+
   static Histogram.Timer startTimer(String key, String... labels) {
     if (Metrics.enabled()) {
       Histogram histogram = container.get(key);

framework/src/main/java/org/tron/core/metrics/blockchain/BlockChainMetricManager.java

@@ -175,11 +175,10 @@ public void applyBlock(BlockCapsule block) {
           MetricLabels.Counter.TXS_SUCCESS, MetricLabels.Counter.TXS_SUCCESS);
     }
 
-    // Empty block detection
-    if (block.getTransactions().isEmpty()) {
-      Metrics.counterInc(MetricKeys.Counter.BLOCK_EMPTY, 1,
-          MetricLabels.Counter.BLOCK_EMPTY);
-    }
+    // Record transaction count distribution for all blocks (including empty blocks)
+    int txCount = block.getTransactions().size();
+    Metrics.histogramObserve(MetricKeys.Histogram.BLOCK_TRANSACTION_COUNT, txCount,
+        StringUtil.encode58Check(address));
 
     // SR set change detection
     List<ByteString> currentSrList =

framework/src/test/java/org/tron/core/metrics/prometheus/PrometheusApiServiceTest.java

@@ -177,6 +177,7 @@ private BlockCapsule createTestBlockCapsule(long time,
   @Test
   public void testEmptyBlockMetric() throws Exception {
     ECKey ecKey = ECKey.fromPrivate(privateKey);
+    String minerBase58 = org.tron.common.utils.StringUtil.encode58Check(ecKey.getAddress());
     ByteString witnessAddress = ByteString.copyFrom(ecKey.getAddress());
 
     chainBaseManager.getWitnessScheduleStore().saveActiveWitnesses(new ArrayList<>());
@@ -192,19 +193,19 @@ public void testEmptyBlockMetric() throws Exception {
     block.sign(privateKey);
 
     Double beforeValue = CollectorRegistry.defaultRegistry.getSampleValue(
-        "tron:block_empty_total",
-        new String[]{"type"},
-        new String[]{"empty"});
+        "tron:block_transaction_count_bucket",
+        new String[]{"miner", "le"},
+        new String[]{minerBase58, "0.0"});
     double before = beforeValue == null ? 0.0 : beforeValue;
 
     blockChainMetricManager.applyBlock(block);
 
     Double afterValue = CollectorRegistry.defaultRegistry.getSampleValue(
-        "tron:block_empty_total",
-        new String[]{"type"},
-        new String[]{"empty"});
-    Assert.assertNotNull("tron:block_empty_total counter should exist", afterValue);
-    Assert.assertEquals("Counter should have incremented by 1",
+        "tron:block_transaction_count_bucket",
+        new String[]{"miner", "le"},
+        new String[]{minerBase58, "0.0"});
+    Assert.assertNotNull("Empty block bucket should exist for miner: " + minerBase58, afterValue);
+    Assert.assertEquals("Histogram bucket le=0.0 should have incremented by 1",
         before + 1.0, afterValue, 0.001);
   }