Add group-by-label feature

The idea of this feature is to limit which groups get selected for
review on a given PR. It adds a match tag to groups with the
effect that if the PR has labels, only groups whose matcher is
present in one of these labels get selected for reviews.

The use case for this is multiplatform or multiteam monorepos
to not get all the groups in an organization reviewing every single
PR.

src/config.ts

@@ -4,6 +4,7 @@ import fs from 'fs'
 
 interface Group {
   name: string
+  matchLabel?: string
   reviewers?: number
   internal_reviewers?: number
   usernames: string[]

src/lottery.ts

@@ -6,6 +6,7 @@ export interface Pull {
   user: {login: string} | null
   number: number
   draft?: boolean
+  labels: {name: string}[] | null
 }
 interface Env {
   repository: string
@@ -79,17 +80,22 @@ class Lottery {
       for (const {
         reviewers,
         internal_reviewers: internalReviewers,
-        usernames
+        usernames,
+        matchLabel
       } of this.config.groups) {
-        const reviewersToRequest =
-          usernames.includes(author) && internalReviewers
-            ? internalReviewers
-            : reviewers
-
-        if (reviewersToRequest) {
-          selected = selected.concat(
-            this.pickRandom(usernames, reviewersToRequest, selected.concat(author))
-          )
+        const labels = this.getPRLabels()
+        const shouldRunForGroup = matchLabel === undefined || labels.length == 0 || labels.includes(matchLabel)
+        if (shouldRunForGroup) {
+          const reviewersToRequest =
+            usernames.includes(author) && internalReviewers
+              ? internalReviewers
+              : reviewers
+
+          if (reviewersToRequest) {
+            selected = selected.concat(
+              this.pickRandom(usernames, reviewersToRequest, selected.concat(author))
+            )
+          }
         }
       }
     } catch (error: any) {
@@ -138,6 +144,16 @@ class Lottery {
     return Number(this.pr?.number)
   }
 
+  getPRLabels(): string[] {
+    const labels = this.pr?.labels?.map((label) => label.name)
+
+    if (labels) {
+      return labels
+    } else {
+      return []
+    } 
+  }
+
   async getPR(): Promise<Pull | undefined> {
     if (this.pr) return this.pr