Rust: Wrap labels in Cow

This allows us to generate non-generic code to turn strings into the
correct type.

glean_parser/metrics.py

@@ -364,14 +364,38 @@ def __init__(self, *args, **kwargs):
         )
 
 
+class CowString(str):
+    """
+    Wrapper class for strings that should be represented
+    as a `Cow<'static, str>` in Rust,
+    or `String` in other target languages.
+
+    This wraps `str`, so unless `CowString` is specifically
+    handled it acts (and serializes)
+    as a string.
+    """
+
+    def __init__(self, val: str):
+        self.inner: str = val
+
+    def __eq__(self, other):
+        return self.inner == other.inner
+
+    def __hash__(self):
+        return self.inner.__hash__()
+
+    def __lt__(self, other):
+        return self.inner.__lt__(other.inner)
+
+
 class Labeled(Metric):
     labeled = True
 
     def __init__(self, *args, **kwargs):
         labels = kwargs.pop("labels", None)
         if labels is not None:
             self.ordered_labels = labels
-            self.labels = set(labels)
+            self.labels = set([CowString(label) for label in labels])
         else:
             self.ordered_labels = None
             self.labels = None

glean_parser/rust.py

@@ -61,6 +61,9 @@ def iterencode(self, value):
                 yield "]"
             elif value is None:
                 yield "None"
+            # `CowStr` is a `str`, so needs to be before next case
+            elif isinstance(value, metrics.CowString):
+                yield f'::std::borrow::Cow::from("{value.inner}")'
             elif isinstance(value, str):
                 yield f'"{value}".into()'
             elif isinstance(value, metrics.Rate):