Feature request - public `registry._names_to_collectors` access

Now we are working on Prometheus support in FastStream and faced with a problem. Shared registry between different middlewares leads to MetricsContainer duplication. So, we would like to have a public API to chech registry for already registered objects.

Can you please provide public access to this property some way?

https://github.com/prometheus/client_python/blob/master/prometheus_client/registry.py#L31

I think, the easiest way to save old API and provide us with a new one - add a public property like in the following snipper

class CollectorRegistry(Collector):
    def __init__(self):
        self._names_to_collectors: Dict[str, Collector] = {}

    @property
    def names_to_collectors(self):
        return self._names_to_collectors

从 issue 中链接的 prometheus_client/registry.py 里的 CollectorRegistry 及其 _names_to_collectors 属性开始。检查 registry 的使用方式，并确定公开访问的兼容性影响。完成的标准是提供一种受支持的公开方式来检查已注册对象，同时不移除现有的内部属性。