No Quota Separation in Transport Layer

### How do you use Sentry?

Sentry Saas (sentry.io)

### Version

2.9.0

### Steps to Reproduce

1. Exhaust performance units quota


2. Attempt to send performance metrics, and you will receive rate limiting issues:
```
[sentry] WARNING: Rate-limited via x-sentry-rate-limits
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 1
```

3. Trigger an error event, it will incorrectly be impacted by the rate limits
```
[sentry] INFO: event processor (.processor at 0xffffa2193560>) dropped event
```

### Expected Result

The Sentry SDK needs a more specific approach to handling rate limits for different event types, such that performance metrics are rate limited when the quota is expended, but error events are still sent to Sentry.

### Actual Result

```
KeyError: 'TriggerTestEvent'
[sentry] INFO: event processor (.processor at 0xffffa2193560>) dropped event
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 2
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 3
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 4
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 5
[sentry] DEBUG: [Monitor] health check negative, downsampling with a factor of 6
```