Added SpanFilter

[marcingrzejszczak]

without this change we cannot filter out spans to be sent to Zipkin
with this change we give users a hook to achieve that

fixes gh-1033

[codecov-io]

Codecov Report

Merging #1037 into master will increase coverage by 0.05%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##             master    #1037      +/-   ##
============================================
+ Coverage     70.87%   70.93%   +0.05%     
- Complexity      637      642       +5     
============================================
  Files           121      121              
  Lines          2819     2828       +9     
  Branches        300      301       +1     
============================================
+ Hits           1998     2006       +8     
  Misses          626      626              
- Partials        195      196       +1

Impacted Files	Coverage Δ	Complexity Δ
...loud/sleuth/autoconfig/TraceAutoConfiguration.java	91.66% <92.85%> (-0.65%)	20 <7> (+5)

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[codefromthecrypt]

I really like this, but I think firehose mode in brave will land before sleuth 2.1. Let's try to do what we need with this. openzipkin/brave#557

Probably will be something like..

boolean beforeReport(context, spanRecord);

or something where you can skip out like you are now, but you can also mutate the record. This would be invoked regardless of whether the data goes to something like micrometer or zipkin

[marcingrzejszczak]

Dropping in favour of Brave changes