Metric units should use UCUM case sensitive variant

[jack-berg]

Related to #705.

[reyang]

Although not introduced by this PR, I just want to call out that we might run into trouble with the current spec:

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-unit

[jack-berg]

@reyang I don't understand - single quote is a valid ascii character, no?

[reyang]

@reyang I don't understand - single quote is a valid ascii character, no?

"single quote is a valid ascii character" - right. The problem I'm seeing - UCUM c/s doesn't say anything about the allowed characters, we don't know if we'll see non-ASCII chars.

[MrAlias]

Although not introduced by this PR, I just want to call out that we might run into trouble with the current spec:

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-unit

Aren't these just two single quote characters (ASCII code 39)?

[MrAlias]

https://ucum.org/ucum#section-Character-Set-and-Lexical-Rules

[reyang]

Ah, I see, that removes the concern. Thanks!