perf: parse trace_id hex without allocating substrings

Author: marcsanmi

lib/src/main/java/io/otel/pyroscope/PyroscopeOtelSpanProcessor.java

@@ -63,9 +63,10 @@ public void onStart(Context parentContext, ReadWriteSpan span) {
 
         span.setAttribute(ATTRIBUTE_KEY_PROFILE_ID, strProfileId);
         asprof.setTracingContext(spanId, spanName);
+        // W3C trace ID is 32 hex chars (128 bits). Parse directly into two longs
+        // to avoid the String#substring allocations on this hot path.
         String traceId = span.getSpanContext().getTraceId();
-        asprof.setTraceId(Long.parseUnsignedLong(traceId.substring(0, 16), 16),
-                          Long.parseUnsignedLong(traceId.substring(16, 32), 16));
+        asprof.setTraceId(parseHex64(traceId, 0), parseHex64(traceId, 16));
     }
 
     @Override
@@ -88,6 +89,25 @@ public static long parseSpanId(String strProfileId) {
         }
     }
 
+    static long parseHex64(String s, int offset) {
+        long result = 0L;
+        for (int i = 0; i < 16; i++) {
+            int c = s.charAt(offset + i);
+            int nibble;
+            if (c >= '0' && c <= '9') {
+                nibble = c - '0';
+            } else if (c >= 'a' && c <= 'f') {
+                nibble = c - 'a' + 10;
+            } else if (c >= 'A' && c <= 'F') {
+                nibble = c - 'A' + 10;
+            } else {
+                throw new NumberFormatException("invalid hex char in trace_id at index " + (offset + i));
+            }
+            result = (result << 4) | nibble;
+        }
+        return result;
+    }
+
     public static boolean isRootSpan(ReadableSpan span) {
         SpanContext parent = span.getParentSpanContext();
         boolean noParent = parent == SpanContext.getInvalid();