Do we need a Gauge<u64>?

[utpilla]

OTLP proto only allows for f64 and i64 values. We already have Gauge<f64> and Gauge<i64>.

Gauge<u64> doesn't add any additional value.

Counters and Histograms could have also used only f64 and i64 variants. However, the spec states that Counter and Histogram measurements should not be negative. So, using an u64 variant, helps enforcing that.

[cijothomas]

Gauge<u64> maybe removed. I understand we are already at a RC state, but this is better to be done before 1.0 after which it is not possible to remove at all.