Add otlp-env-vars skill for OpenTelemetry SDK env var configuration

.github/skills/otlp-env-vars/SKILL.md

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+---
+name: otlp-env-vars
+description: Configure and troubleshoot OpenTelemetry SDK environment variables safely.
+---
+
+# Skill: Configure OpenTelemetry SDK via Environment Variables
+
+Use this skill when configuring, reviewing, or troubleshooting OpenTelemetry SDK environment variables.
+
+Source: OpenTelemetry Environment Variable Specification, OTel 1.56.0: https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/
+
+## Core rules
+
+- Treat an empty environment variable exactly like an unset variable.
+- Boolean values are true only when set to `"true"` case-insensitively; all other values are false.
+- Invalid numeric values should be warned about and ignored.
+- Enum values should be case-insensitive; unknown enum values must warn and be ignored.
+
+## Common variables
+
+### General SDK
+
+- `OTEL_SDK_DISABLED`: disable SDK when `true`
+- `OTEL_SERVICE_NAME`: sets `service.name`; overrides `service.name` in `OTEL_RESOURCE_ATTRIBUTES`
+- `OTEL_RESOURCE_ATTRIBUTES`: comma-separated resource attributes
+- `OTEL_LOG_LEVEL`: SDK internal log level, default `info`
+- `OTEL_PROPAGATORS`: default `tracecontext,baggage`
+- `OTEL_TRACES_SAMPLER`: default `parentbased_always_on`
+- `OTEL_TRACES_SAMPLER_ARG`: argument for selected sampler
+
+### Exporter selection
+
+- `OTEL_TRACES_EXPORTER`: default `otlp`
+- `OTEL_METRICS_EXPORTER`: default `otlp`
+- `OTEL_LOGS_EXPORTER`: default `otlp`
+
+Valid common exporter values include `otlp`, `console`, `none`; traces also support `zipkin`, and metrics support `prometheus`. `logging` is deprecated.
+
+### Batch processors
+
+Span processor:
+
+- `OTEL_BSP_SCHEDULE_DELAY`: default `5000`
+- `OTEL_BSP_EXPORT_TIMEOUT`: default `30000`
+- `OTEL_BSP_MAX_QUEUE_SIZE`: default `2048`
+- `OTEL_BSP_MAX_EXPORT_BATCH_SIZE`: default `512`
+
+LogRecord processor:
+
+- `OTEL_BLRP_SCHEDULE_DELAY`: default `1000`
+- `OTEL_BLRP_EXPORT_TIMEOUT`: default `30000`
+- `OTEL_BLRP_MAX_QUEUE_SIZE`: default `2048`
+- `OTEL_BLRP_MAX_EXPORT_BATCH_SIZE`: default `512`
+
+### Limits
+
+- `OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT`
+- `OTEL_ATTRIBUTE_COUNT_LIMIT`: default `128`
+- `OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT`: default `128`
+- `OTEL_SPAN_EVENT_COUNT_LIMIT`: default `128`
+- `OTEL_SPAN_LINK_COUNT_LIMIT`: default `128`
+- `OTEL_LOGRECORD_ATTRIBUTE_COUNT_LIMIT`: default `128`
+
+### Metrics
+
+- `OTEL_METRICS_EXEMPLAR_FILTER`: default `trace_based`
+- `OTEL_METRIC_EXPORT_INTERVAL`: default `60000`
+- `OTEL_METRIC_EXPORT_TIMEOUT`: default `30000`
+
+### Declarative config
+
+- `OTEL_CONFIG_FILE`: path to SDK config file
+- If set, all other SDK environment variables are ignored unless referenced through environment-variable substitution in the config file.
+
+## Troubleshooting checklist
+
+1. Check whether `OTEL_CONFIG_FILE` is set; it overrides flat SDK env vars.
+2. Check empty values; they behave as unset.
+3. Validate booleans: only `true` enables true behavior.
+4. Validate enum spelling: unsupported values are ignored.
+5. Check signal-specific exporter settings: traces, metrics, and logs are configured independently.
+6. Confirm SDK/language support; implementations may choose whether to support these env vars.