Tracking: spec-required environment variables not yet supported

[bryantbiggs]

Summary

Cross-referencing the SDK environment variable specification and OTLP exporter specification against the current implementation reveals several spec-required environment variables that are not yet supported and have no PR in flight. This issue tracks them in one place.

The recently merged #3358 documents what is supported today. This issue covers the gaps.

Environment variables with active PRs (for reference)

These are already being worked on and are not blocked:

• OTEL_SDK_DISABLED — [Feature]: add support for environment variable OTEL_SDK_DISABLED #1936 / PR feat: add support for OTEL_SDK_DISABLED environment variable #3088
• OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT — PR feat: add support for OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT #3090
• OTEL_EXPORTER_OTLP_TRACES_PROTOCOL / OTEL_EXPORTER_OTLP_METRICS_PROTOCOL / OTEL_EXPORTER_OTLP_LOGS_PROTOCOL — PR feat(otlp): support per-signal protocol environment variables #3363
• OTEL_EXPORTER_OTLP_*_CERTIFICATE / OTEL_EXPORTER_OTLP_*_CLIENT_CERTIFICATE / OTEL_EXPORTER_OTLP_*_CLIENT_KEY — PR feat(otlp): support TLS certificate environment variables #3364
• OTEL_EXPORTER_OTLP_*_INSECURE — PR feat(otlp): support INSECURE environment variables for gRPC #3365

Environment variables NOT yet supported (no PR in flight)

SDK — General / Attribute Limits

• OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT — global fallback for all signals; per-span variant is in PR feat: add support for OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT #3090 but the global limit is not addressed
• OTEL_ATTRIBUTE_COUNT_LIMIT — global fallback for all signals; not implemented at all

SDK — Propagators

• OTEL_PROPAGATORS — [Feature]: add support for OTEL_PROPAGATORS environment variable #3127 exists but has no PR; also related to [Feature]: Support the environment variable propagation spec in Rust #3285

SDK — Log Record Limits

• OTEL_LOGRECORD_ATTRIBUTE_VALUE_LENGTH_LIMIT — no issue or PR
• OTEL_LOGRECORD_ATTRIBUTE_COUNT_LIMIT — no issue or PR

SDK — Metrics / Exemplars

• OTEL_METRICS_EXEMPLAR_FILTER — blocked on exemplar implementation (Implement exemplar collection for metrics (required for stable) #3369); spec values are always_on, always_off, trace_based

OTLP Exporter — Metrics

• OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION — spec values are explicit_bucket_histogram, base2_exponential_bucket_histogram; no issue or PR

SDK — Exporter Selection

• OTEL_TRACES_EXPORTER / OTEL_METRICS_EXPORTER / OTEL_LOGS_EXPORTER — previously discussed and closed as out-of-scope ([Feature]: OTEL_TRACES_EXPORTER support #3197) since it would require an uber crate bundling all exporters. Not required for stable but listed here for completeness. Supporting none to disable a signal would still be valuable and could be decoupled from the full exporter-selection logic.

Notes

• The global attribute limits (OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT, OTEL_ATTRIBUTE_COUNT_LIMIT) are meant to be fallbacks when per-signal limits are not set. They apply across spans, log records, and (where applicable) events and links. See the spec.
• The log record limits are the logs equivalent of the span limits that already exist (OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT, etc.).
• OTEL_METRICS_EXEMPLAR_FILTER is gated on the broader exemplar implementation tracked in Implement exemplar collection for metrics (required for stable) #3369.
• Some variables (exporter selection) may not be practical for this project's architecture, but they are part of the spec and are noted here for transparency.