feat: Add a warning message for scheduled deprecation of Coherence me…

…tric class

PiperOrigin-RevId: 658500900

vertexai/preview/evaluation/metrics/_coherence.py

@@ -15,16 +15,25 @@
 # limitations under the License.
 #
 from typing import Optional
+import warnings
+
 from vertexai.preview.evaluation import constants
 from vertexai.preview.evaluation.metrics import _base
 
+_DEPRECATION_WARNING_MESSAGE = (
+    "After google-cloud-aiplatform>1.63.0, using metric class `Coherence` will"
+    " result in an error. Please use string metric name `coherence` or define"
+    " a PointwiseMetric instead."
+)
+
 
 class Coherence(_base._ModelBasedMetric):
     """The model-based pointwise metric for Coherence."""
 
     _metric_name = constants.Metric.COHERENCE
 
     def __init__(self, *, version: Optional[int] = None):
+        warnings.warn(message=_DEPRECATION_WARNING_MESSAGE)
         super().__init__(
             metric=Coherence._metric_name,
             version=version,