fix: Add null handling to PrometheusFormatter.escapeLabelValue()

Fixes #205

Added null check to escapeLabelValue() method to prevent NullPointerException
when label maps contain null values. Returns empty string for null values
as Prometheus doesn't support null label values.

Changes:
- escapeLabelValue(): Check if value is null before calling replace() methods
- Return empty string for null values (Prometheus compatible)
- Updated javadoc to document null handling behavior
- Added test testLabelValueNull() to verify null labels are handled correctly
- Test confirms null values are converted to empty strings without errors

This prevents metrics export failures when applications have null metadata
and ensures all metrics are successfully formatted.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Flossy

jplatform-metrics-prometheus/src/main/java/org/flossware/jplatform/metrics/prometheus/PrometheusFormatter.java

@@ -121,10 +121,13 @@ private static String formatLabels(Map<String, String> labels) {
      * Escapes special characters in label values according to Prometheus format.
      * Escapes backslashes, double quotes, and newlines.
      *
-     * @param value the label value to escape
-     * @return escaped label value
+     * @param value the label value to escape (may be null)
+     * @return escaped label value, or empty string if value is null
      */
     private static String escapeLabelValue(String value) {
+        if (value == null) {
+            return "";  // Prometheus doesn't allow null values, use empty string
+        }
         return value
                 .replace("\\", "\\\\")  // Escape backslashes first
                 .replace("\"", "\\\"")  // Escape double quotes

jplatform-metrics-prometheus/src/test/java/org/flossware/jplatform/metrics/prometheus/PrometheusFormatterTest.java

@@ -143,4 +143,20 @@ void testCompleteMetricFormatting() {
         assertTrue(result.contains("state=\"running\""));
         assertTrue(result.contains(" 1.0\n"));
     }
+
+    @Test
+    void testLabelValueNull() {
+        Map<String, String> labels = new HashMap<>();
+        labels.put("app_id", "test-app");
+        labels.put("version", null);  // null label value
+
+        // Should not throw NullPointerException, null should be converted to empty string
+        String result = PrometheusFormatter.formatGauge("my_metric", labels, 1.0);
+
+        assertNotNull(result);
+        assertTrue(result.startsWith("my_metric{"));
+        assertTrue(result.contains("app_id=\"test-app\""));
+        assertTrue(result.contains("version=\"\""));  // null becomes empty string
+        assertTrue(result.endsWith(" 1.0\n"));
+    }
 }