Decouple the trace id struct generation from its encoding

[oleiade]

This PR was triggered by this comment from @mstoykov. It intends to refactor the trace id structure, and its encoding method in a way that decouples them from each other.

Trace ids are serialised as 16 byte slices, which are in turn encoded into hexadecimal strings that are attached to requests HTTP headers. Some parts of the trace id have a fixed size, some others, a varying one (time, and random suffix). Because some of its parts have a varying size, we need to start serializing it to find out how much space is the random suffix gonna take.

Thus, in order to decouple the trace id creation from its encoding, we have made the trace id struct private (traceId) private, and exposed a trace id constructor (NewTraceID) which returns an interface object implementing an interface that offers the Encode() (string, error) method. That way, when the constructor is called, the serialization happens on the fly, and its result is stored in an internal field.

🦖

[codecov-commenter]

Codecov Report

Merging #3019 (6f7f984) into master (14d80f6) will decrease coverage by 0.01%.
The diff coverage is 88.23%.

❗ Current head 6f7f984 differs from pull request most recent head ead38b6. Consider uploading reports for the commit ead38b6 to get more accurate results

@@            Coverage Diff             @@
##           master    #3019      +/-   ##
==========================================
- Coverage   77.00%   77.00%   -0.01%     
==========================================
  Files         228      228              
  Lines       17066    17057       -9     
==========================================
- Hits        13141    13134       -7     
+ Misses       3082     3081       -1     
+ Partials      843      842       -1     

Flag	Coverage Δ
ubuntu	77.00% <88.23%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
js/modules/k6/experimental/tracing/client.go	69.44% <75.00%> (-1.36%)	⬇️
js/modules/k6/experimental/tracing/trace_id.go	86.66% <92.30%> (+7.71%)	⬆️

... and 3 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[codebien]

LGTM, just one comment

[mstoykov]

I think I was misunderstood.

As I go longer in comments - my main point was that the creation and the encoding are needlessly separated. There is nothing to do with a trace id apart from encoding it, so we can just get that as a single operation (newTraceID) that just returns the encoded traceid as a string. And forgo all the other types, methods and structs.

[oleiade]

You were misunderstood indeed @mstoykov 😄
Those are fair points 👍🏻 Dropping the Encode method indeed.

[mstoykov]

LGTM!