detect: read OTEL_IGNORE_ERROR from environment - #6966

What this fixes

Commit 228e250d moved OTEL_IGNORE_ERROR handling into detectExporter, but the
refactor passed the environment variable name directly to strconv.ParseBool instead
of reading its value. Parsing the literal string always fails, so exporter errors cannot
be ignored even when OTEL_IGNORE_ERROR=true.

This restores the environment lookup used before that refactor.

Changes

• read OTEL_IGNORE_ERROR from the environment;
• add regression coverage for trace and metric exporters;
• verify both the default error behavior and explicit error suppression.

The test uses an unknown exporter name, so it does not invoke a detector or connect to
an external telemetry backend.

Validation

go test ./util/tracing/detect/...
go vet ./util/tracing/detect/...
gofmt -l util/tracing/detect/

All tests and vet pass, and gofmt -l produces no output.

Behavior note

Operators with OTEL_IGNORE_ERROR already enabled will regain the error-suppression
behavior that existed before the refactor.