[SES-PHASE-1-8] - PLU-744: add bounce rate tracking (exploration)

[m0nggh]

TL;DR

Add DogStatsD metrics tracking for SES email events (sent, bounced, complained).

WHY???

• AWS reviews/suspends senders whose bounce rate stays above ~5/10% or complaint rate above ~0.5%.
• SES's own CloudWatch reputation metrics are account-level, not per-configuration-set.
• We can track and monitor internally since we have full context of whether an email is bounced or not

What changed?

A new incrementMetric helper wraps the DogStatsD tracer client to provide a reusable way to emit counter metrics. This helper is now called in three places:

• When an email is successfully sent via SES (in both email-helper.ts and ses-email-helper.ts), emitting ses.email.sent
• When a bounce event is processed, emitting ses.email.bounce tagged with bounce_type and bounce_sub_type
• When a complaint event is processed, emitting ses.email.complaint tagged with complaint_feedback_type

How to test?

To test locally, DogStatsD writes UDP packets to 127.0.0.1:8125. Sniff with nc:

# Terminal A — listen
nc -ul 8125

# Terminal B — start app
cd packages/backend && npm run dev

Tried testing on uat

• Trigger a transactional email send and verify the ses.email.sent metric appears in Datadog
• Simulate or observe a bounce/complaint SES event being processed and verify the corresponding metrics appear with the correct tags in Datadog
• Run the existing test suite, which has been updated to mock the new incrementMetric helper

Why make this change?

Visibility into SES email delivery health was limited. By tracking sent, bounce, and complaint counts as metrics in Datadog, we can monitor delivery rates, detect spikes in bounces or complaints, and set up alerts without relying solely on logs.

[m0nggh]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [SES-PHASE-1-9] - PLU-744: send attachments with ses #1636
• [SES-PHASE-1-8] - PLU-744: add bounce rate tracking (exploration) #1634 👈 (View in Graphite)
• [SES-PHASE-1-7] - PLU-744: add env vars for ses and sqs #1633
• [SES-PHASE-1-6] - PLU-744: add admin whitelist endpoint #1632
• [SES-PHASE-1-5] - PLU-744: add suppression check for ses send email #1631
• [SES-PHASE-1-4] - PLU-744: add consumer for sqs events #1630
• [SES-PHASE-1-3] - PLU-744: add ses event parser #1629
• [SES-PHASE-1-2] - PLU-744: add migration and email suppression model #1628
• [SES-PHASE-1-1] - PLU-744: add SES call and LD flag #1627
• feat/ses/trunk

This stack of pull requests is managed by Graphite. Learn more about stacking.

[linear]

PLU-744

[ogp-weeloong]

nit: technically we can avoid multiple calls to this and just increment by total number of successfully sent emails inside the postman action itself.

But keeping it here is also ok, i dont think there is any material difference

[ogp-weeloong]

lgtm