Add Warn internal logging function

[pellared]

Ended up not creating a Warn function as mentioned in the issue since Info is already being used for warning messages in other places.

Originally from #3854

We should consider adding Warn logging function to go.opentelemetry.io/otel/internal/global.

[unicod3]

I can work on this one but have a question, Is there any agreed verbosity level for the Warn?

As far as I see we currently have Debug (v-level: 5) and Info (v-level: 1), Ideally verbosity level of warn should be lower than info since it is meant to be more important than info.

One approach could be changing the verbosity level of info close to debug, but I am not sure about implications of it.

[MrAlias]

I can work on this one but have a question, Is there any agreed verbosity level for the Warn?

As far as I see we currently have Debug (v-level: 5) and Info (v-level: 1), Ideally verbosity level of warn should be lower than info since it is meant to be more important than info.

One approach could be changing the verbosity level of info close to debug, but I am not sure about implications of it.

Can we map to a pseudo inverse of what the OTel logging spec is: https://github.com/open-telemetry/opentelemetry-specification/blob/c10191c1bf2a6961145f3c5c1b16b89d54af02c0/specification/logs/data-model.md#field-severitynumber

For example:

Name	LogR Severity	OTel Severity
Error	0	17-24
Warn	1	13-16
Info	4	9-12
Debug	8	5-8

[MadVikingGod]

The reason that the original values were chosen was to somewhat match expectations of current uses of logr.

Most uses of logr on github, only even use level 0. A few will use level 1 with a -v flag. And there were one or two uses of 5 for debug-level logging.