[Metrics SDK] Switch to explicit 64 bit integers

[sam-leitch-oxb]

All "Long" instruments in the API accept long as an input value. long does not have a well defined bit depth and can represent either 64 bit or 32 bit integers depending on the compiler implementation (and configuration).

"Long" instruments should be changed to accept int64_t integer values, which are unambiguously 64 bit depth as requested by the API specification: https://opentelemetry.io/docs/reference/specification/common/#attribute

[lalitb]

Specs don't say anything about the type/size of the measurement value. The above link is explicitly for attribute values. Ideally, we can support both fixed (32-bit, 64-bit) and architecture-dependent (long, int) data types at the API surface. At the SDK level, the generated metrics can be stored as a 64-bit integer.

[sam-leitch-oxb]

You're right. My mistake. I can't find anything that explicitly requires specific value types.

My goal was only to prevent unintended integer overflow on 32 bit compilers, since counters that pass the 32bit boundary are not-uncommon (eg. timestamps, memory usage, network bytes read/written)

[lalitb]

My goal was only to prevent unintended integer overflow on 32 bit compilers, since counters that pass the 32bit boundary are not-uncommon (eg. timestamps, memory usage, network bytes read/written)

Yes, this is a valid concern, we should fix this at the API level. Thanks for raising the issue.