[SDK] Read span limits from environment variables#4258
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Adds span_limits_env::GetSpanLimitsFromEnv(), which builds a SpanLimits from the environment variables defined in the specification (OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT, OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT, OTEL_SPAN_EVENT_COUNT_LIMIT, OTEL_SPAN_LINK_COUNT_LIMIT, OTEL_EVENT_ATTRIBUTE_COUNT_LIMIT, OTEL_LINK_ATTRIBUTE_COUNT_LIMIT, and the general OTEL_ATTRIBUTE_* fallbacks), and uses it as the default in the TracerProviderFactory::Create overloads that previously defaulted to SpanLimits::NoLimits(). A field whose variable is unset keeps its NoLimits() value, so behavior is unchanged when no variables are set; explicitly passed SpanLimits keep taking precedence.
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Fixes #4257
Changes
Adds
span_limits_env::GetSpanLimitsFromEnv(), which builds aSpanLimitsfrom the environment variables defined in the specification, and uses it as the default in theTracerProviderFactory::Createoverloads that previously defaulted toSpanLimits::NoLimits(). The implementation mirrors the existingbatch_span_processor_options_envpattern (OTEL_BSP_*).Semantics, chosen to preserve the backward-compatible default established in #4232:
SpanLimits::NoLimits()value — with no variables set,GetSpanLimitsFromEnv() == SpanLimits::NoLimits()and SDK behavior is unchanged.OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT,OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT,OTEL_SPAN_EVENT_COUNT_LIMIT,OTEL_SPAN_LINK_COUNT_LIMIT,OTEL_EVENT_ATTRIBUTE_COUNT_LIMIT,OTEL_LINK_ATTRIBUTE_COUNT_LIMIT, plus the generalOTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT/OTEL_ATTRIBUTE_COUNT_LIMIT, with the span-specific variables taking precedence per the spec.SpanLimits(programmatic, or declarative configuration through the sdk builder) continue to take precedence over the environment, matching the spec's configuration-precedence order.Follow-up once merged: flip the C++ cells of the span-limit env-variable rows in the spec compliance matrix (separate PR to the specification repo).
For significant contributions please make sure you have completed the following items:
CHANGELOG.mdupdated for non-trivial changes