Add cassandra index filter

[black-adder]

Signed-off-by: Won Jun Jang wjang@uber.com

Which problem is this PR solving?

• There are certain times when you want to skip indexing a certain span. ie for debug spans, the most common use case is that the user will use the traceid directly to find the trace, there's no need to index it by duration, etc.

Short description of the changes

• Add a new IndexFilter which checks the span and determines whether it should be indexed or not. Defaults to indexing everything

[codecov]

Codecov Report

Merging #876 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #876   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         125    126    +1     
  Lines        6046   6057   +11     
=====================================
+ Hits         6046   6057   +11

Impacted Files	Coverage Δ
...ugin/storage/cassandra/spanstore/writer_options.go	100% <100%> (ø)	⬆️
plugin/storage/cassandra/spanstore/writer.go	100% <100%> (ø)	⬆️
...torage/cassandra/spanstore/dbmodel/index_filter.go	100% <100%> (ø)

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[isaachier]

This is OK, just not a huge fan of the hard coded function names. Constants can accomplish the same thing dynamically:

const (
    durationIndex = iota
    serviceIndex
    operationIndex
)

type IndexFilter func(span *Span, index int)

[yurishkuro]

fair point

[jpkrohling]

Perhaps I missed it, but would the possibility of filtering out debug spans come in a later PR?

[jpkrohling]

This filter will filter out everything, right? Would it be more appropriate to name it noNameFilter{}?

[isaachier]

@jpkrohling this mechanism could be used to implement that feature as well.

type debugFilter struct
func (debugFilter) IndexByX(span *Span) bool {
    return !span.Context().(jaeger.SpanContext).IsDebug()
}

[isaachier]

LGTM

[black-adder]

@jpkrohling later PR, I promise