refactor(metrics): extract MINER_LABEL constant for histogram labels

Extract repeated 'miner' string literal into MINER_LABEL constant
in MetricsHistogram to follow DRY principle and fix SonarQube warning.

Also update PrometheusApiServiceTest to use MINER_LABEL constant
for test assertions.

Relates tronprotocol#6624

Author: warku123

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

@@ -9,6 +9,7 @@
 public class MetricsHistogram {
 
   private static final Map<String, Histogram> container = new ConcurrentHashMap<>();
+  private static final String MINER_LABEL = "miner";
 
   static {
     init(MetricKeys.Histogram.INTERNAL_SERVICE_LATENCY, "Internal Service latency.",
@@ -20,7 +21,7 @@ public class MetricsHistogram {
     init(MetricKeys.Histogram.JSONRPC_SERVICE_LATENCY, "JsonRpc Service latency.",
         "method");
     init(MetricKeys.Histogram.MINER_LATENCY, "miner latency.",
-        "miner");
+        MINER_LABEL);
     init(MetricKeys.Histogram.PING_PONG_LATENCY, "node  ping pong  latency.");
     init(MetricKeys.Histogram.VERIFY_SIGN_LATENCY, "verify sign latency for trx , block.",
         "type");
@@ -36,7 +37,7 @@ public class MetricsHistogram {
     init(MetricKeys.Histogram.PROCESS_TRANSACTION_LATENCY, "process transaction latency.",
         "type", "contract");
     init(MetricKeys.Histogram.MINER_DELAY, "miner delay time, actualTime - planTime.",
-        "miner");
+        MINER_LABEL);
     init(MetricKeys.Histogram.UDP_BYTES, "udp_bytes traffic.",
         "type");
     init(MetricKeys.Histogram.TCP_BYTES, "tcp_bytes traffic.",
@@ -52,7 +53,7 @@ public class MetricsHistogram {
     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");
+        MINER_LABEL);
   }
 
   private MetricsHistogram() {

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

@@ -40,6 +40,9 @@
 
 @Slf4j(topic = "metric")
 public class PrometheusApiServiceTest extends BaseTest {
+
+  private static final String MINER_LABEL = "miner";
+
   static LocalDateTime localDateTime = LocalDateTime.now();
   @Resource
   private DposSlot dposSlot;
@@ -87,7 +90,7 @@ protected void check(byte[] address, Map<ByteString, String> witnessAndAccount)
     // Query histogram bucket le="0.0" for empty blocks
     Double emptyBlock = CollectorRegistry.defaultRegistry.getSampleValue(
         "tron:block_transaction_count_bucket",
-        new String[] {"miner", "le"}, new String[] {minerBase58, "0.0"});
+        new String[] {MINER_LABEL, "le"}, new String[] {minerBase58, "0.0"});
 
     Assert.assertNotNull("Empty block bucket should exist for miner: " + minerBase58, emptyBlock);
     Assert.assertEquals("Should have 1 empty block", 1, emptyBlock.intValue());
@@ -125,7 +128,7 @@ protected void check(byte[] address, Map<ByteString, String> witnessAndAccount)
       // Collect empty blocks count from histogram bucket
       Double witnessEmptyBlock = CollectorRegistry.defaultRegistry.getSampleValue(
           "tron:block_transaction_count_bucket",
-          new String[] {"miner", "le"}, new String[] {witnessBase58, "0.0"});
+          new String[] {MINER_LABEL, "le"}, new String[] {witnessBase58, "0.0"});
       Assert.assertNotNull("Empty block bucket should exist for witness: " + witnessBase58,
           witnessEmptyBlock);
       totalNewWitnessEmptyBlocks += witnessEmptyBlock;