Add metric from/to protobuf v1alpha8 conversion

To make sure we can decouple the clients from the underlying protocol,
we need to make sure we always use explicit conversion between Python
enums and protobuf enums. This is especially important to be able to
support multiple versions of a protobuf message, for example an upcoming
v1alpha9 Metric enum, so downstream project can migrate to new versions
in a backwards-compatible way.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

RELEASE_NOTES.md

@@ -4,13 +4,17 @@
 
 <!-- Here goes a general summary of what this release is about -->
 
-## Upgrading
+## Deprecation
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- Converting `Metric` enums from/to protobuf directly is deprecated and will be dropped in the next breaking release.
+
+    You should switch to use the new conversion functions in `frequenz.client.common.metrics.proto.v1alpha8` to convert from/to protobuf.
+
+    Since we can't emit deprecation messages for this (as they will trigger every time a metric value is used), please consider using the new conversion functions as soon as possible so the migration to the next breaking release is smooth.
 
 ## New Features
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+- A new module `frequenz.client.common.metrics.proto.v1alpha8` has been added to provide conversion functions for `Metric`s from/to protobuf version `v1alpha8`.
 
 ## Bug Fixes
 

src/frequenz/client/common/metrics/proto/v1alpha8/__init__.py

@@ -0,0 +1,11 @@
+# License: MIT
+# Copyright © 2026 Frequenz Energy-as-a-Service GmbH
+
+"""Conversion of Metric from/to protobuf v1alpha8."""
+
+from ._metric import metric_from_proto, metric_to_proto
+
+__all__ = [
+    "metric_from_proto",
+    "metric_to_proto",
+]

src/frequenz/client/common/metrics/proto/v1alpha8/_metric.py

@@ -0,0 +1,34 @@
+# License: MIT
+# Copyright © 2026 Frequenz Energy-as-a-Service GmbH
+
+"""Coversion of Metric to/from protobuf v1alpha8."""
+
+
+from frequenz.api.common.v1alpha8.metrics import metrics_pb2
+
+from ....proto import enum_from_proto
+from ..._metric import Metric
+
+
+def metric_from_proto(message: metrics_pb2.Metric.ValueType) -> Metric | int:
+    """Convert a protobuf Metric message to a Metric enum member.
+
+    Args:
+        message: A protobuf Metric message.
+
+    Returns:
+        The corresponding Metric enum member.
+    """
+    return enum_from_proto(message, Metric)
+
+
+def metric_to_proto(metric: Metric) -> metrics_pb2.Metric.ValueType:
+    """Convert a Metric enum member to a protobuf Metric message.
+
+    Args:
+        metric: A Metric enum member.
+
+    Returns:
+        The corresponding protobuf Metric message.
+    """
+    return metrics_pb2.Metric.ValueType(metric.value)