Delay evaluation of resource.Default to first call

[bmon]

Currently defaultResource (which includes data calculated by parsing env var OTEL_RESOURCE_ATTRIBUTES) is evaluated as an import side effect. Some applications might want to update that variable with go code (eg to load extra env vars from a configuration file, for example using https://github.com/joho/godotenv). By only evaluating it the first time resource.Default() is called, we can allow those applications an opportunity to set OTEL_RESOURCE_ATTRIBUTES first, while still only calculating the value once.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Brendan Roy (f975a15)

[codecov]

Codecov Report

Merging #2371 (d39a348) into main (2fa7bce) will decrease coverage by 0.0%.
The diff coverage is 57.1%.

@@           Coverage Diff           @@
##            main   #2371     +/-   ##
=======================================
- Coverage   73.8%   73.8%   -0.1%     
=======================================
  Files        175     175             
  Lines      12430   12439      +9     
=======================================
+ Hits        9184    9185      +1     
- Misses      3012    3017      +5     
- Partials     234     237      +3     

Impacted Files	Coverage Δ
sdk/resource/resource.go	82.5% <57.1%> (-1.2%)	⬇️
sdk/metric/refcount_mapped.go	80.0% <0.0%> (-20.0%)	⬇️
sdk/metric/sdk.go	80.2% <0.0%> (-1.5%)	⬇️
exporters/jaeger/jaeger.go	94.3% <0.0%> (+0.8%)	⬆️

[WillsonHG]

/easycla

[MrAlias]

The implementation looks good to me. Can you add an entry to the changelog describing the changed behavior to the end user?

[bmon]

Thanks very much, all. Cheers! 👍