Abstract key lookup into a trait for Family::get_or_create about client_rust

[cheshirskycode]

@mxinden, could you take a look? I'd like to implement it If it sounds reasonable

[mxinden]

Thank you for the code snippets. Generally I am not opposed to moving to hashbrown. That said, I am not yet fully sure I understand the rational.

Are you trying to optimize performance? If so, would you mind providing data on how much the above would safe you?

Are you trying to optimize ergonomics? If so, I don't think having to implement Equivalent is any better than calling to_string().

[cheshirskycode]

I have huge label sets (10+ items) and can't formalize them in static types, it's just Strings in general. I have &str slices during data processing and copying them to locate the counter doesn't look good.

I've added some bench in PR:
The best option is counter family with custom type label set. It doesn't contain allocations and is very cheap for hashing.
counter family with custom type label set and direct lookup search by type with 3 string fields&Labels {method: "GET".to_string(), url_path: "/metrics".to_string(), status_code: "200".to_string()} - 61.247 ns
counter family with custom type label set and equivalent lookup - the same but without copying - 13.324 ns

counter family with Vec<(String, String)> label set
                        time:   [104.90 ns 106.36 ns 107.95 ns]
Found 7 outliers among 100 measurements (7.00%)
  4 (4.00%) high mild
  3 (3.00%) high severe

counter family with custom type label set
                        time:   [6.4575 ns 6.4720 ns 6.4883 ns]
Found 13 outliers among 100 measurements (13.00%)
  8 (8.00%) high mild
  5 (5.00%) high severe

counter family with custom type label set and direct lookup
                        time:   [61.076 ns 61.247 ns 61.422 ns]
Found 9 outliers among 100 measurements (9.00%)
  8 (8.00%) high mild
  1 (1.00%) high severe

counter family with custom type label set and equivalent lookup
                        time:   [13.291 ns 13.324 ns 13.358 ns]

[hdost]

Perhaps using `Cowˋ would be useful?