Allow unit only for the quantity metric

This is implemented the same way time_unit is implemented for others:
The specific metric class takes the argument, not the generic one.

Because the schema enforces `unit` to be required for `quantity` we can
just `pop` it with the guarantee it's there.

The error message for other metrics if they do set `unit` is less nice:

    Metric.__init__() got an unexpected keyword argument 'unit'

But that's in line with what `time_unit` also does.

CHANGELOG.md

@@ -3,6 +3,8 @@
 ## Unreleased
 
 - BREAKING CHANE: `ping` lifetime metrics on the events ping are now disallowed ([#625](https://github.com/mozilla/glean_parser/pull/625))
+- Disallow `unit` field for anything but quantity ([#630](https://github.com/mozilla/glean_parser/pull/630)).
+  Note that this was already considered the case, now the code enforces it.
 
 ## 9.0.0
 

glean_parser/metrics.py

@@ -55,7 +55,6 @@ def __init__(
         disabled: bool = False,
         lifetime: str = "ping",
         send_in_pings: Optional[List[str]] = None,
-        unit: Optional[str] = None,
         gecko_datapoint: str = "",
         no_lint: Optional[List[str]] = None,
         data_sensitivity: Optional[List[str]] = None,
@@ -86,8 +85,6 @@ def __init__(
         if send_in_pings is None:
             send_in_pings = ["default"]
         self.send_in_pings = send_in_pings
-        if unit is not None:
-            self.unit = unit
         self.gecko_datapoint = gecko_datapoint
         if no_lint is None:
             no_lint = []
@@ -237,6 +234,10 @@ class Counter(Metric):
 class Quantity(Metric):
     typename = "quantity"
 
+    def __init__(self, *args, **kwargs):
+        self.unit = kwargs.pop("unit")
+        Metric.__init__(self, *args, **kwargs)
+
 
 class TimeUnit(enum.Enum):
     nanosecond = 0

tests/test_parser.py

@@ -1039,3 +1039,18 @@ def test_no_internal_fields_exposed():
         indent=2,
     )
     assert expected_json == out_json
+
+
+def test_unit_not_accepted_on_nonquantity():
+    results = parser.parse_objects(
+        [
+            util.add_required(
+                {
+                    "category": {"metric": {"type": "counter", "unit": "quantillions"}},
+                }
+            ),
+        ]
+    )
+    errs = list(results)
+    assert len(errs) == 1
+    assert "got an unexpected keyword argument 'unit'" in str(errs[0])