Missing Channel ID could be clearer

### Environment

SaaS ([https://sentry.io/]())

### Steps to Reproduce

1. Create a new metric alert, or edit an existing one
2. Add an action with Discord as the target
3. Leave the channel ID blank
4. Save Rule

### Expected Result

The empty field should be highlighted, or the error should state which specific field is missing. Or do both.

### Actual Result

When saving a metric alert with a discord action and no channel ID, the error thrown is not clear which field is missing


### Product Area

Alerts

### Link

*No response*

### DSN

*No response*

### Version

*No response*

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)*