Add level metadata to spans

[mTsBucy1]

See #180 for a description.

This PR implements the OpenTelemetryLayer::with_level setting, to disable (enabled by default) attaching the level tag to exported spans.

There are several concerns I see before recommending merging this:

• Will enable by default break code which manually set the level field? If so, disable-by-default seems smarter.
• Event levels are still always exported, should we also restrict this?
• I am somewhat confused, but it seems error level events set the span status to error. Is this behaviour intuitive, once we have a level attached to the span? Should error spans always set the span status?

[djc]

@mladedav any opinions on this? I don't have a strong opinion.

[mladedav]

Looks good to me and I think this will be useful.

This should just also include a changelog entry for anyone who sets some kind of level explicitly as a span field.

[djc]

@mladedav thanks for the quick followup!