[docstore] add metrics for writes to docs collection (#31741)

* [docstore] add metrics for writes to docs collection

Co-authored-by: Brian Gough <brian.gough@overleaf.com>

* [docstore] fix import

* [docstore] use counter metric

* [docstore] add metric on lines delta

Co-authored-by: Brian Gough <brian.gough@overleaf.com>

---------

Co-authored-by: Brian Gough <brian.gough@overleaf.com>
GitOrigin-RevId: 4662c5862315c800cb7b4d57bef60e08fbe7ba59

services/docstore/test/unit/js/MongoManager.test.js

@@ -1,9 +1,11 @@
 import sinon from 'sinon'
-import { ObjectId } from 'mongodb-legacy'
+import mongodb from 'mongodb-legacy'
 import path from 'node:path'
 import { assert, beforeEach, describe, expect, it, vi } from 'vitest'
 import Errors from '../../../app/js/Errors.js'
 
+const { ObjectId, BSON } = mongodb
+
 const modulePath = path.join(
   import.meta.dirname,
   '../../../app/js/MongoManager'
@@ -22,6 +24,7 @@ describe('MongoManager', () => {
       default: {
         db: ctx.db,
         ObjectId,
+        BSON,
       },
     }))