Singleton `Metrics.All` is mutable

[YohDeadfall]

The idea of any singleton to be immutable if it serves an idea of a constant value, but unfortunately, it doesn't apply to Metric exposing values as a HashSet<string>:

elasticsearch-net/src/Elastic.Clients.Elasticsearch/Core/Infer/Metric/Metrics.cs

Line 22 in 9e58d38

public static Metrics All { get; } = new("_all");

A potential solution is to use an immutable collection or IReadOnlySet interface which limits mutability, but that can be allowed through methods added to Metrics.