feat(discover): add custom perf metric unit to measurements-meta response (#38748)

Adds the custom performance metric unit to measurements-meta response

src/sentry/api/endpoints/organization_measurements_meta.py

@@ -33,6 +33,7 @@ class OrganizationMeasurementsMeta(OrganizationEventsEndpointBase):  # type: ign
             result = {
                 item["name"]: {
                     "functions": METRIC_FUNCTION_LIST_BY_TYPE[item["type"]],
+                    "unit": item["unit"],
                 }
                 for item in metric_meta
             }

tests/snuba/api/endpoints/test_organization_measurements_meta.py

@@ -65,6 +65,7 @@ class OrganizationMeasurementsMetaEndpoint(MetricsEnhancedPerformanceTestCase):
                     "sum",
                     "percentile",
                 ],
+                "unit": "millisecond",
             }
         }