## Bug description

Since upgrading from 1.36.1 to 1.37.0, I've been seeing a high volume of errors in Sentry with the message "Object of type Decimal is not JSON serializable." The trace indicates that this is caused by capturing internal metrics, specifically from [`_capture` in `posthog/internal_metrics/__init__.py`](https://github.com/PostHog/posthog/blob/1.37.0/posthog/internal_metrics/__init__.py#L36). An example of an event that failed to serialize is

```python
{
  event: '$$pg_table_cache_hit_rate', 
  properties: {
    table: 'posthog_cohort', 
    value: Decimal('99.59125055236411842700')
  }
}
```

It seems the Postgres query added in #10392 is returning `Decimal`s and, although [`statshog` doesn't mind this](https://github.com/macobo/statshog/blob/master/statshog/client/base.py#L73), it doesn't work when dumping the metrics into Kafka as JSON.

Two possible solutions I see:
- Convert the `Decimal` to a float [here](https://github.com/PostHog/posthog/blob/97c327e3923943b5c226b058d59487a232d624fe/posthog/celery.py#L194):
```diff
- gauge("pg_table_cache_hit_rate", row[1], tags={"table": row[0]})
+ gauge("pg_table_cache_hit_rate", float(row[1]), tags={"table": row[0]})
```
- Subclass [`JSONEncoder`](https://docs.python.org/3/library/json.html#json.JSONEncoder) to handle the `Decimal` type and pass it [here](https://github.com/PostHog/posthog/blob/97c327e3923943b5c226b058d59487a232d624fe/posthog/api/capture.py#L50):
```diff
- "data": json.dumps(data),
+ "data": json.dumps(data, cls=NewJsonEncoder),
```

Because `json.dumps` is used in a variety of places, updating it to support decimals in just one place may cause surprising behavior if decimals end up elsewhere in PostHog, so my bias is towards simply casting the result of the query to a float. Another option would be to change _all_ uses of `json.dumps` to use a new encoder, but that adds future overhead that any new additions of `json.dumps` also need to remember to use the new encoder or else risk inconsistency. I'll wait for guidance from someone at PostHog before choosing a path forward and making a PR.

## How to reproduce

1. Set the `CAPTURE_INTERNAL_METRICS` environment variable to `"true"`

## Environment

- [ ] PostHog Cloud
- [X] self-hosted PostHog (ClickHouse-based), version/commit: 1.37.0
- [ ] self-hosted PostHog (Postgres-based, legacy), version/commit: _please provide_
posthog/celery.py
from posthog.tasks.verify_persons_data_in_sync import verify_persons_data_in_sync as verify
    verify()
def recompute_materialized_columns_enabled() -> bool:
    from posthog.models.instance_setting import get_instance_setting
    if get_instance_setting("MATERIALIZED_COLUMNS_ENABLED") and get_instance_setting(
        "COMPUTE_MATERIALIZED_COLUMNS_ENABLED"
    ):
        return True
    return False
@app.task(ignore_result=True)
def clickhouse_materialize_columns():
    if recompute_materialized_columns_enabled():
        try:
            from ee.clickhouse.materialized_columns.analyze import materialize_properties_task
        except ImportError:
            pass
        else:
            materialize_properties_task()
@app.task(ignore_result=True)
def clickhouse_mark_all_materialized():
    if recompute_materialized_columns_enabled():
        try:
            from ee.tasks.materialized_columns import mark_all_materialized
        except ImportError:
            pass
        else:
            mark_all_materialized()
@app.task(ignore_result=True)
def clickhouse_send_license_usage():