Change dbmodel.TagFilter to take span as argument

[yurishkuro]

Because sometimes the filtering decision depends not only on the tags, but on the span itself, e.g. debug bit, or the service name.

Signed-off-by: Yuri Shkuro ys@uber.com

[vprithvi]

I'm not a fan of this approach because the interface is essentially accepting the same information twice.

Perhaps having a variant that takes in only the full span instead of the both the span and tags could work out cleaner

[black-adder]

Chaining becomes a bit funky but I think we can get it to work

[black-adder]

Alternative, keep the interface as is but have db.Model span implement it

[yurishkuro]

Alternative, keep the interface as is but have db.Model span implement it

I need to control the implementation of the filter. I cannot change the implementation of the span when doing the filtering. So how would that help? Also, filters are static.

[coveralls]

Coverage remained the same at 100.0% when pulling d6dc147 on storage-tag-filter-takes-span into 59cc364 on master.