Avoid allocations for single label value

[avendel]

One of the WithLabels overloads uses a params argument. Calling a method with a single value for a params argument allocates memory, and I suspect that using a single label is quite common.

This change will prevent that for the common scenario of having only one label, by introducing a non-allocating method.

Benchmark results

| Method                                   | Mean       | Error   | StdDev  | Allocated |
|----------------------------------------- |-----------:|--------:|--------:|----------:|
| WithLabels_OneMetric_ManySeries_OneLabel | 1,046.9 ns | 4.94 ns | 4.12 ns |     320 B |
| WithLabel_OneMetric_ManySeries_OneLabel  |   984.5 ns | 5.13 ns | 4.80 ns |         - |