Support autologging span attributes as loki labels

[mapno]

PR Description

Add support autologging span attributes as log labels. This skips the need to format the log line and allows to directly search via labels. But as a downside, it increases cardinality.

Which issue(s) this PR fixes

Fixes #661

Notes to the Reviewer

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[mapno]

I'm not completely sold on this approach. It depends on the attribute/tag to be logged in the log line, duplicating data. It also does not support to add default values.

I wanted to make it as simple as possible to configure, but maybe we can make it a bigger feature.

[joe-elliott]

map[string]struct{} is i think the "right" way to do a flag map like this

[mapno]

map[string]struct{} uses less memory since an empty struct is 0 bytes, but it's very similar in performance to map[string]bool (see https://gist.github.com/davecheney/3be245c92b61e5045f75). I think map[string]bool makes for a bit clearer code.

[rfratto]

FWIW I try to use map[string]struct{} whenever I can, because there's always a chance that a bug could creep in that sets an entry to false in the map which would be treated as if it didn't exist.

[mapno]

That's a good point, changed in 7efae7e

[joe-elliott]

if k.(string) is guaranteed to not panic can we just take a []string instead?

[mapno]

Changed the logic a bit to make it cleaner and safer.

[mapno]

@rfratto @joe-elliott could you please take a second look?

[joe-elliott]

one nit. nice work.

this will be nice in our infra. everytime i query this log stream i wish there were more filters.