`RequestMetrics.update_metrics()` and `_BackgroundProcess.update_metrics()` can result in negative counts

`RequestMetrics.update_metrics()` and `_BackgroundProcess.update_metrics()` can result in negative counts.

### `RequestMetrics.update_metrics()`

Sentry error: https://sentry.tools.element.io/organizations/element/issues/11196863
```
ValueError: Counters can only be incremented by non-negative amounts.
```

Relevant code:

[mypy `Counter.inc(...)` error](https://github.com/prometheus/client_python/blob/73680284ce63f0bc0f23cfc42af06e74fd7e3ccf/prometheus_client/metrics.py#L286-L290)

https://github.com/element-hq/synapse/blob/e16fbdcdcc95d626f6d3dcd0b6a85fdb0a06456a/synapse/http/request_metrics.py#L278-L299

https://github.com/element-hq/synapse/blob/e16fbdcdcc95d626f6d3dcd0b6a85fdb0a06456a/synapse/logging/context.py#L184-L191

### `_BackgroundProcess.update_metrics()`

Occurrences in Sentry but for the background process metrics:

- https://sentry.tools.element.io/organizations/element/issues/11196776
- https://sentry.tools.element.io/organizations/element/issues/289

Relevant code:

https://github.com/element-hq/synapse/blob/19fe3f001ed0aff5a5f136e440ae53c04340be88/synapse/metrics/background_process_metrics.py#L182-L188

---

This is a historical error before any of the [recent changes to metrics](https://github.com/element-hq/synapse/issues/18592).