Use SSO in `SharedString`

[loyd]

Shouldn't name/labels use SmartString inside? Almost all dynamic label values in my production fit in 23 bytes. Also, values of u64 also fit in 23 bytes. IPv4+port also fits into 23 bytes.

metrics already have custom Cow implementation, what about altering the Owned type to SmartString?

[tobz]

I've never gotten around to benchmarking it, but I wouldn't be against such a change since it does seem likely to provide a benefit in cases of short strings.