Bug fix: Prevent exemplar drop when filtered attributes exceed 128-rune limit

[NesterovYehor]

I’ve added a test case for #6718 to show what happens when filtered attributes are too long.

Currently, the test fails because the Prometheus client_golang has a 128-rune limit for exemplars. When we move filtered attributes into the exemplar and they exceed this limit, the whole thing gets dropped, including the trace_id and span_id. It even causes a parsing error in the output:

--- FAIL: TestExemplars (0.00s)
    --- FAIL: TestExemplars/exemplar_overflow_does_not_drop_exemplar (0.00s)
        exporter_test.go:1405:
            Error Trace:    /Users/yehornesterov/dev/Go/opentelemetry-go/exporters/prometheus/exporter_test.go:1405
            Error:          Expected value not to be nil.
            Test:           TestExemplars/exemplar_overflow_does_not_drop_exemplar
FAIL
FAIL    go.opentelemetry.io/otel/exporters/prometheus    0.373s

To fix this, I can add a check that ensures we always fit the trace_id and span_id first. Then, we only add the user's filtered attributes if there is enough space left in the 128-rune budget. If an attribute doesn't fit, we just skip it so we don't lose the Trace ID.

[dashpole]

Thanks for adding the test. You will need to add the fix as well for us to merge the PR.

[NesterovYehor]

okey, I started by adding the test to clearly capture the failure case and expected behavior. I’ll add the fix next so we can get this merged.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.7%. Comparing base (24567b3) to head (ab3eec2).

Files with missing lines	Patch %	Lines
exporters/prometheus/exporter.go	89.4%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7883     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        304     304             
  Lines      23283   23296     +13     
=======================================
+ Hits       19034   19043      +9     
- Misses      3862    3865      +3     
- Partials     387     388      +1     

Files with missing lines	Coverage Δ
exporters/prometheus/exporter.go	90.6% <89.4%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes

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[dashpole]

My only concern with this is that we (Prometheus) are planning to relax the exemplar limit in OM 2.0: prometheus/docs#2835. I'm not sure if we would be able to remove this code without causing problems for people still using OM 1.0.

@bwplotka do you think we should contribute this fix to the prometheus client instead so that it can have different exemplar behavior for OM 1.0 vs 2.0?