Notify user when a new identity is linked or removed from their account

### Problem Statement

Currently, in Sentry, when a user links a new Identity or removes an identity (ex. Discord, Google, Slack, etc) to their account there is no email notification.

Linked identities can be found under `/settings/account/identities/`.

### Solution Brainstorm

Send an email notification to the primary verified email on the account regarding the identity linking/removal.

### Product Area

Settings - Auth

We want to define a set of [lightweight metrics]() that SDKs can automatically collect for e.g. framework, language or platform, focusing only on signals that **do NOT overlap with Tracing**. Ideally these metrics should give a quick sense of app or runtime health, … while deeper investigation will still rely on traces or profiles.

Examples**:**

* Node.js runtime metrics (e.g., event loop delay, heap usage)
* Prisma-related metrics in Next.js
* Mobile CPU / memory usage to help surface potential ANRs or app hangs

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**Candidate Metrics (add more below)**

* …

*(please also add ideas, questions or examples in comments)*