Warn about time adjustment in tags

[bobrik]

Time adjustment is confusing to many people, as you can see in #961.

This change adds a warning if we do any time adjustments, so that it's at least clear that adjustments happened.

[bobrik]

I spent a few hours trying to figure out how on earth can a span for 301 redirect follow can happen before the first request that triggered said redirect finished.

The reason was that tracing with futures is hard and redirect following span outlived its parent, which triggered an adjustment. This adjustment may or may not be a good idea, but let's at least be clear it happened, so people can save some time in the future:

[codecov]

Codecov Report

Merging #2052 into master will decrease coverage by 0.81%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2052      +/-   ##
==========================================
- Coverage   98.21%   97.39%   -0.82%     
==========================================
  Files         195      207      +12     
  Lines        9602    10307     +705     
==========================================
+ Hits         9431    10039     +608     
- Misses        134      224      +90     
- Partials       37       44       +7

Impacted Files	Coverage Δ
model/adjuster/clockskew.go	100% <100%> (ø)	⬆️
cmd/agent/app/reporter/flags.go	68.75% <0%> (-31.25%)	⬇️
cmd/agent/app/flags.go	92.3% <0%> (-7.7%)	⬇️
pkg/queue/bounded_queue.go	93.58% <0%> (-6.42%)	⬇️
...gin/storage/cassandra/spanstore/operation_names.go	97.59% <0%> (-2.41%)	⬇️
cmd/collector/app/span_processor.go	98.43% <0%> (-1.57%)	⬇️
plugin/storage/cassandra/spanstore/reader.go	100% <0%> (ø)	⬆️
cmd/collector/app/builder/span_handler_builder.go	100% <0%> (ø)	⬆️
cmd/agent/app/reporter/tchannel/collector_proxy.go	100% <0%> (ø)	⬆️
cmd/collector/app/metrics.go	100% <0%> (ø)	⬆️
... and 56 more

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[yurishkuro]

I don't think we should use span.tags for that, but rather span.warnings, which we are already using for some messages about clock skew adjustment.

The URL doesn't seem to be necessary.

Please add a unit test to check for the expected outcome.

[bobrik]

It definitely looks better as a warning, updated:

[yurishkuro]

Thanks!

[prongs]

Any traction on this?