refactor(BA-5878): clarify legacy-direction in ValueType label helpers

The two helpers were named "to_live_stat_label" / "from_legacy_live_stat_label",
which obscured their actual asymmetry: only the encoder produces the
legacy "stats.<name>" form, while the decoder accepts both legacy and
current shapes. Rename them to:

- ValueType.to_legacy_live_stat_label — encoder, legacy emission
- ValueType.from_live_stat_label — decoder, accepts either form

and document the "stats." prefix as the legacy convention so the
removeprefix branch reads as historical compatibility, not generic
parsing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: seedspirit

src/ai/backend/common/clients/prometheus/fixed_query_builder.py

@@ -211,7 +211,7 @@ def _render_stats_query(
         group_by: str,
     ) -> str:
         stat_fn = f"{bucket.value_type.value}_over_time"
-        stat_label = bucket.value_type.to_live_stat_label()
+        stat_label = bucket.value_type.to_legacy_live_stat_label()
         parts: list[str] = []
         if bucket.gauge_metrics or bucket.gauge_metric_patterns:
             gauge_regex = _metric_name_regex(bucket.gauge_metrics, bucket.gauge_metric_patterns)

src/ai/backend/common/clients/prometheus/metric_types.py

@@ -201,7 +201,7 @@ def from_prometheus_response(cls, response: PrometheusResponse) -> Self:
             container_metric_name = cast(str, info.container_metric_name)
             value_type_str = cast(str, info.value_type)
             try:
-                value_type = ValueType.from_legacy_live_stat_label(value_type_str)
+                value_type = ValueType.from_live_stat_label(value_type_str)
                 kernel_id = KernelId(UUID(kernel_id_str))
             except ValueError:
                 continue

src/ai/backend/common/clients/prometheus/types.py

@@ -12,12 +12,14 @@ class ValueType(StrEnum):
     AVG = "avg"
 
     @classmethod
-    def from_legacy_live_stat_label(cls, value: str) -> "ValueType":
-        if value.startswith("stats."):
+    def from_live_stat_label(cls, value: str) -> "ValueType":
+        if value.startswith(
+            "stats."
+        ):  # Legacy live_stat labels were prefixed with "stats." (e.g. "stats.max", "stats.avg")
             return cls(value.removeprefix("stats."))
         return cls(value)
 
-    def to_live_stat_label(self) -> str:
+    def to_legacy_live_stat_label(self) -> str:
         match self:
             case ValueType.MAX | ValueType.AVG:
                 return f"stats.{self.value}"