feat: Add baggageItemToTag filter

[addityasingh]

Add baggageItemToTag filter

This filter adds an opentracing tag for a given baggage item in the trace.

Syntax:

baggageItemToTag("<baggage_item_name>", "<tag_name>")

Example: If a trace consists of baggage item named foo with a value bar. Adding below filter will add a tag named baz with value bar

baggageItemToTag("foo", "baz")

[szuecs]

@addityasingh thanks for the PR, can you please also add something in the documentation?
docs/reference/filters.md would need an update

[aryszka]

👍

[szuecs]

👍

[mikkeloscar]

Is baggage a general term used in OpenTracing? Just looking at the filter name baggageItemToTag I had no idea it had anything to do with OpenTracing but maybe it's just a convention that I'm not familiar with?

It's documented so I can learn that it's an OpenTracing thing but ideally filter names should be somewhat descriptive on their own.

[szuecs]

See https://godoc.org/github.com/opentracing/opentracing-go#Span it’s a member of this interface

[addityasingh]

Baggage is indeed a term used in opentracing to represent information valid throughout the process boundaries, unlike Span or Logs https://opentracing.io/docs/overview/tags-logs-baggage/

[aryszka]

thx @mikkeloscar . I think I get your concern. From Skipper's perspective it may not be necessary clear, what kind of 'baggage' this suppose to be. My suggestion now is to rename it to: tracingBaggageToSpan. Wdyt?

[szuecs]

@aryzska that is even more weird to me.
We copy from span.BagageItem to span.Tag.
Do I miss something?

[addityasingh]

IMO the name is self explanatory if we consider that BaggageItem and Tags are part of the core components of opentracing(https://opentracing.io/docs/overview/tags-logs-baggage/)

[szuecs]

I agree, but maybe we should link in our docs to the opentracing docs

[lmineiro]

I'm late to this party. For consistency, it probably would've been better to prefix with tracing just like the filter tracingSpanName

[mikkeloscar]

Thanks I understand now that there were some meaningful reason behind the name in the context of open tracing, however I would still argue that we should avoid using such names in a "global" namespace of filter names. It's not unlikely that other parts of the system could have similar terminology internally as OpenTracing. I agree with @lmineiro that having it prefixed with tracing would make sense, similar to how the oauth filters also have the oauth prefix :)

[aryszka]

prefixing was my intention, too, while keeping it short. Maybe a good name would be: tracingBaggageToTag (@szuecs you're right, tag is better than span)

[addityasingh]

Cool. I will update the name and create a PR later today with tracingBaggageToTag

[addityasingh]

@mikkeloscar This PR(#1168) addresses the renaming