oss_26_declarative_config_env_substitution

[ocelotl]

Declarative config env substitution: support the env: prefix form, apply default-value operator when a var is set-but-empty, and wire top-level attribute_limits/log_level into configure_sdk (Linear OSS-26, findings L5/L6/L7).

Branch / worktree: oss_26_declarative_config_env_substitution

Linear issue: https://linear.app/dash0/issue/OSS-26/l5-l6-l7-declarative-config-env-substitution-top-level-attribute

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-26/l5-l6-l7-declarative-config-env-substitution-top-level-attribute (findings L5/L6/L7)
• Slug/branch/worktree: oss_26_declarative_config_env_substitution
• Commit: 70bd8450c · PR: oss_26_declarative_config_env_substitution #36
• Validation: opentelemetry-configuration suite 374 passed; scope package + changelog.
• Reviewer note / cross-link: top-level attribute_limits is applied only to spans; the SDK LoggerProvider takes no programmatic attribute limits, so logs/metrics are not covered and configure_sdk warns. This is exactly the gap tracked by OSS-21 (L2) — once L2 lands, the logs path can be wired here.

[ocelotl]

📣 Public-facing draft — to be opened upstream in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Declarative config: ${env:VAR} unsupported, :- default ignored for empty vars, and top-level log_level/attribute_limits are no-ops

The declarative configuration implementation in opentelemetry-configuration has three defects in env-var references and top-level fields.

1. Prefixed reference form ${env:VAR} is not substituted

src/opentelemetry/configuration/file/_env_substitution.py — the substitution regex \$\$|\$\{([A-Za-z_][A-Za-z0-9_]*)(:-([^}]*))?\} has no group for the env: prefix, so ${env:SERVICE_NAME} passes through unsubstituted. The data-model reference grammar allows an optional env: prefix; ${env:VAR} must behave identically to ${VAR}. Fix: add an optional non-capturing prefix group (?:env:)?.

2. ${VAR:-default} default not applied when the variable is set-but-empty

replace_var applies the default only when the var is unset (is None); a set-but-empty value substitutes empty instead of the default. Spec (configuration/data-model.md): the default applies when the variable is null, empty, or undefined. Fix: treat set-but-empty like unset for applying a provided default (a bare ${VAR} with no default and an empty value still substitutes empty; only a truly undefined var with no default is an error).

3. Top-level log_level and attribute_limits parsed but never applied

models.py defines log_level and attribute_limits on OpenTelemetryConfiguration, but _sdk.py configure_sdk never reads them — both are silent no-ops. Fix: apply log_level to the SDK internal logger and attribute_limits where supported. Note: the SDK LoggerProvider constructor accepts no attribute limits and there is no global attribute-limits hook, so attribute_limits can currently be applied only to spans (via SpanLimits); the limitation should be surfaced (e.g. a warning) rather than silently dropped for logs/metrics.