this commit introduces a `len()` method to `Family<S, M, C>`, which
returns the number of series within a metric family.

see also prometheus#245, which allows callers to check if a family `contains()` a
given label set.

Signed-off-by: katelyn martin <[email protected]>

```shell
$ cargo clippy --all-features --all-targets --tests
   Compiling prometheus-client v0.23.0 (/path/to/prometheus-client)
warning: struct `Family` has a public `len` method, but no `is_empty` method
   --> src/metrics/family.rs:309:5
    |
309 |     pub fn len(&self) -> usize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty
    = note: `#[warn(clippy::len_without_is_empty)]` on by default

warning: `prometheus-client` (lib) generated 1 warning
warning: `prometheus-client` (lib test) generated 1 warning (1 duplicate)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.63s
```

Signed-off-by: katelyn martin <[email protected]>