traceIDRatioSampler: use rightmost bits

[danielmmetz]

Before this PR:

The traceIDRatioSampler, when used with the xray.IDGenerator, does not make random sampling decisions.
In detail, the xray.IDGenerator generates trace.TraceIDs such that the first 4 bytes represent a timestamp, with the remaining 12 bytes being randomly generated.
The traceIDRatioSampler uses the first 8 bytes to make sampling decisions which means in practice any provided ratio ends up being effectively 0 or 1.0 for any given point in time.

Stated differently, for (nearly) any fixed point in time any provided ratio will be (effectivley ignored) and the sampler will consistently choose to either drop all traces or to record all traces, with no mix between.

After this PR:

The traceIDRatioSampler uses the last 8 bytes of the trace.TraceID, which lines up with the fully random portion of the ID generated by xray.IDGenerator. Also brings the Go implementation into better alignment with its Python and Java equivalents (code references linked later).

Demo

Check out https://go.dev/play/p/qpxm-boZuxj. Notably, at a sample rate of 0.25, I get sampled 0/10000 (0.000000%) but for a sample rate of 0.5, I get sampled 10000/10000 (100.000000%).

Alternatives considered

The existing traceIDRatioSampler's implementation is correct under the assumption that the first values for 8 bytes are distributed uniformly randomly. With that in mind, we could consider this to be a bug with the ID generator for breaking this requirement, or just conclude the two are incompatible. In that direction, I think we'd expect the xray package to provide a ratio sampler of its own, (and to call out clearly in documentation that it's incompatible with this package's ratio sampler).

However,

1. anecdotally, embedding non-random data into the leftmost bits of IDs during generation doesn't seem uncommon.
2. the Python and Java implementations of the TraceIDRatioSampler also use the rightmost bits of the trace IDs for decision making.

• Python: bitmap and thresholding site.
• Java: offset to use the right half of the ID.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: danielmmetz / name: Daniel Metz (57736df)

[codecov]

Codecov Report

Merging #3557 (fa78ac7) into main (69e44a3) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3557   +/-   ##
=====================================
  Coverage   78.0%   78.1%           
=====================================
  Files        164     164           
  Lines      11815   11815           
=====================================
+ Hits        9226    9232    +6     
+ Misses      2393    2387    -6     
  Partials     196     196           

Impacted Files	Coverage Δ
sdk/trace/sampling.go	100.0% <100.0%> (ø)
sdk/metric/periodic_reader.go	91.3% <0.0%> (+1.2%)	⬆️
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+1.7%)	⬆️

[hanyuancheung]

👍🏻

[danielmmetz]

I appreciate the reviews and merge, thanks! Would you also be able to share an estimate for when the next release will be cut?