add support for different label value types

[wazazaby]

With this PR, we can set a bool, float or int as a label value using the corresponding builder methods.

It either uses strconv.Append... functions or append under the hood to be as fast as possible and to make fewer allocations possible.

Running benchmarks gives a solid 1 alloc/op for each test case :)

Example usage

var builder vimebu.Builder
builder.Metric("your_metric_name")
builder.Label("host", 1.2.3.4) // String.
builder.LabelBool("with_retry", true) // Boolean.
builder.LabelInt("val", 1234) // Int.
builder.LabelFloat("p", 123.456) // Float.
_ = builder.String() // your_metric_name{host="1.2.3.4",with_retry="true",val="1234",p="123.456"}