Add PRGAIN enum for AUC metric types

Author: iazzi

keras/src/metrics/metrics_utils.py

@@ -43,17 +43,20 @@ class AUCCurve(Enum):
 
     ROC = "ROC"
     PR = "PR"
+    PRGAIN = "PRGAIN"
 
     @staticmethod
     def from_str(key):
         if key in ("pr", "PR"):
             return AUCCurve.PR
         elif key in ("roc", "ROC"):
             return AUCCurve.ROC
+        elif key in ("prgain", "PRGAIN"):
+            return AUCCurve.PRGAIN
         else:
             raise ValueError(
                 f'Invalid AUC curve value: "{key}". '
-                'Expected values are ["PR", "ROC"]'
+                'Expected values are ["PR", "ROC", "PRGAIN"]'
             )