MEDIUM: otel: wired OTel C wrapper library integration

MEDIUM: otel: wired OTel C wrapper library integration

Connected the OpenTelemetry C wrapper library to the filter lifecycle by
implementing the library initialization, tracer creation, memory and
thread callbacks, shutdown sequence, and span completion.

The flt_otel_lib_init() function now verifies the C wrapper library
version against the compiled headers, calls otelc_init() with the absolute
configuration file path, and creates the tracer via otelc_tracer_create().
On success, it registers HAProxy pool-based memory callbacks
(flt_otel_mem_malloc, flt_otel_mem_free) and a thread ID callback
(flt_otel_thread_id) through otelc_ext_init(), so the C++ SDK allocates
span and context objects from pool_head_otel_span_context.  A custom log
handler (flt_otel_log_handler_cb) is registered via otelc_log_set_handler()
to count OTel SDK internal diagnostic messages in the flt_otel_drop_cnt
counter.

The per-thread init callback now starts the tracer thread via
OTELC_OPS(tracer, start) instead of unconditionally returning success.

The deinit callback saves the tracer handle before freeing the
configuration, then shuts down the library via otelc_deinit() after the
pool is destroyed, ensuring the ext callbacks remain valid while the
configuration structures are still being freed.  In debug builds, it logs
wrapper statistics, attach counters, and per-event HTX usage counters
before shutdown.

The runtime context cleanup in flt_otel_runtime_context_free() now ends
all active spans with a common monotonic timestamp via
OTELC_OPSR(span, end_with_options) before freeing them.  The scope context
cleanup in flt_otel_scope_context_free() now destroys the underlying OTel
span context via OTELC_OPSR(context, destroy).

The parser gained static storage for the debug memory tracker
(OTELC_DBG_MEM) and its initialization in the parse entry point, used when
compiled with the OTELC_DBG_MEM flag.