fix(metrics): :impl Atomic<u64> for AtomicU64

[koushiro]

fix the error when using Gauge<u64, AtomicU64> type, revert the impl Atomic<u64> fro AtomicU64 from #105

[mxinden]

If I recall correctly, I removed this implementation, given that the protobuf definition does not support u64, but only i64. With your patch, how would one encode a u64::MAX in a protobuf?

https://github.com/OpenObservability/OpenMetrics/blob/1386544931307dff279688f332890c31b6c5de36/proto/openmetrics_data_model.proto#L100-L107

[koushiro]

Ok, If the protobuf definition does not support u64, why not add impl Atomic<u32> for AtomicU32, at least we could use Gauge<u32, AtomicU32>

[mxinden]

Ok, If the protobuf definition does not support u64, why not add impl Atomic<u32> for AtomicU32, at least we could use Gauge<u32, AtomicU32>

Sorry for the late reply. I am fine with adding u32 to Gauge @koushiro.