Retriving RateLimiterStatistics from PartitionedRateLimiter without providing a resource

## Background and Motivation

Currently, retrieving RateLimitStatistics from a PartitionedRateLimiter requires specifying the resource for which the statistics are needed. This parameter is used to identify the appropriate rate limiter associated with those resources. However, in certain use cases, it may be desirable to retrieve the statistics for all rate limiters managed by the PartitionedRateLimiter.

## Proposed API

```diff
namespace System.Threading.RateLimiting;

public abstract class PartitionedRateLimiter : IAsyncDisposable, IDisposable
{
+ public abstract RateLimiterStatistics[] GetStatistics();
}
```

## Usage Examples

```csharp
public class RateLimiterHealthCheck : BackgroundService {
private readonly PartitionedRateLimiter _partitionedRateLimiter
public RateLimiterHealthCheck(PartitionedRateLimiter partitionedRateLimiter){
_partitionedRateLimiter = partitionedRateLimiter
}

protected override async Task ExecuteAsync(CancellationToken stoppingToken) {
while(!stoppingToken.IsCancellationRequested) {
foreach(var stats in _partitionedRateLimiter.GetStatistics()){
//Sending measurement
MeasurementSender.Send(stats)
}
}
}
}
```

## Alternative Designs

We attempted to retrieve metrics from metric `Microsoft.AspNetCore.RateLimiting`, but found that the provided data is aggregated, which may not satisfy scenarios requiring more detailed or per-partition insights.

## Risks

There is a possibility of encountering race conditions when accessing the dictionary that holds rate limiter instances.