refactor: centralize telemetry metadata formatting

modules/core/ingest-api/src/main/java/com/lingniu/ingest/api/event/TelemetryMetadataValues.java

@@ -0,0 +1,22 @@
+package com.lingniu.ingest.api.event;
+
+import java.util.Locale;
+
+/**
+ * Stable string formatting for metadata values shared across protocol mappers.
+ */
+public final class TelemetryMetadataValues {
+
+    private TelemetryMetadataValues() {
+    }
+
+    public static String doubleValue(double value) {
+        if (!Double.isFinite(value)) {
+            return "";
+        }
+        String formatted = String.format(Locale.ROOT, "%.6f", value);
+        return formatted.indexOf('.') < 0
+                ? formatted
+                : formatted.replaceAll("0+$", "").replaceAll("\\.$", "");
+    }
+}

modules/core/ingest-api/src/test/java/com/lingniu/ingest/api/event/TelemetryMetadataValuesTest.java

@@ -0,0 +1,23 @@
+package com.lingniu.ingest.api.event;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class TelemetryMetadataValuesTest {
+
+    @Test
+    void formatsDoubleWithoutFloatingPointNoise() {
+        assertThat(TelemetryMetadataValues.doubleValue(123456.70000000001)).isEqualTo("123456.7");
+        assertThat(TelemetryMetadataValues.doubleValue(1234.5)).isEqualTo("1234.5");
+        assertThat(TelemetryMetadataValues.doubleValue(1234.0)).isEqualTo("1234");
+        assertThat(TelemetryMetadataValues.doubleValue(1.23456789)).isEqualTo("1.234568");
+    }
+
+    @Test
+    void rejectsNonFiniteDoubleValuesAsBlankMetadata() {
+        assertThat(TelemetryMetadataValues.doubleValue(Double.NaN)).isEmpty();
+        assertThat(TelemetryMetadataValues.doubleValue(Double.POSITIVE_INFINITY)).isEmpty();
+        assertThat(TelemetryMetadataValues.doubleValue(Double.NEGATIVE_INFINITY)).isEmpty();
+    }
+}