fix(webapp): bound the snapshot-store cohort metric to two labels

The cohort metric labelled each member with its organizationId, so the series
count grew with the cohort and leaked an unbounded high-cardinality id into
metrics. Collapse it to a fixed cohort/other pair; per-org investigation moves
to traces and logs filtered by orgId.

Author: d-cs

apps/webapp/app/v3/cohortMetricLabel.server.ts

@@ -1,8 +1,10 @@
-// Bounds per-org metric cardinality: only cohort members get a distinct label, everything else
-// (undefined included) collapses to "other" — at most cohort size plus one series.
+// Bounds per-org metric cardinality to exactly two series. A cohort member is labelled "cohort" and
+// everything else (undefined included) "other", so the series count never grows with the cohort. Never
+// emit the organizationId itself: it is unbounded at rollout and violates the no-high-cardinality-label
+// rule. Per-org investigation belongs in traces/logs filtered by orgId, not in a metric label.
 export function cohortMetricLabel(
   organizationId: string | undefined,
   isCohortMember: (organizationId: string) => boolean
 ): string {
-  return organizationId !== undefined && isCohortMember(organizationId) ? organizationId : "other";
+  return organizationId !== undefined && isCohortMember(organizationId) ? "cohort" : "other";
 }

apps/webapp/test/cohortMetricLabel.test.ts

@@ -2,8 +2,8 @@ import { describe, expect, it } from "vitest";
 import { cohortMetricLabel } from "~/v3/cohortMetricLabel.server";
 
 describe("cohortMetricLabel", () => {
-  it("returns the org id when it is a cohort member", () => {
-    expect(cohortMetricLabel("org_123", (id) => id === "org_123")).toBe("org_123");
+  it("returns the fixed 'cohort' label for a member, never the org id", () => {
+    expect(cohortMetricLabel("org_123", (id) => id === "org_123")).toBe("cohort");
   });
 
   it("returns 'other' for a non-member org", () => {
@@ -14,9 +14,12 @@ describe("cohortMetricLabel", () => {
     expect(cohortMetricLabel(undefined, () => true)).toBe("other");
   });
 
-  it("collapses every org to 'other' with an always-false predicate", () => {
-    for (const id of ["org_a", "org_b", "org_c"]) {
-      expect(cohortMetricLabel(id, () => false)).toBe("other");
+  it("emits at most two distinct labels regardless of how many orgs are members", () => {
+    const labels = new Set<string>();
+    for (const id of ["org_a", "org_b", "org_c", "org_d"]) {
+      labels.add(cohortMetricLabel(id, () => true));
+      labels.add(cohortMetricLabel(id, () => false));
     }
+    expect([...labels].sort()).toEqual(["cohort", "other"]);
   });
 });