MINOR: otel: test: added test and benchmark suite for the OTel filter

MINOR: otel: test: added test and benchmark suite for the OTel filter

Added a test suite under addons/otel/test/ for the OpenTelemetry filter.
Five scenarios exercise different filter capabilities: standalone (sa)
covers all hook points including idle-timeout heartbeats, metrics and log
records; compact (cmp) covers the full request/response lifecycle with
ACL-based error handling; context (ctx) tests explicit inject/extract
propagation through numbered context variables; frontend/backend (fe/be)
tests distributed tracing across two HAProxy instances; and empty tests
bare filter initialisation with no active scopes.

A performance benchmarking script (test-speed.sh) uses wrk to measure
throughput and latency at different rate-limit settings (100% through 0%,
disabled, and filter-off).  Each scenario includes comprehensive YAML
exporter definitions covering OTLP file/gRPC/HTTP, ostream, memory,
Zipkin, and Elasticsearch backends.