MINOR: otel: changed log-record attr to use sample expressions

MINOR: otel: changed log-record attr to use sample expressions

Replaced the static key-value attribute storage in log-record with
sample-evaluated attributes.  The 'attr' keyword now accepts a key and a
HAProxy sample expression evaluated at runtime, instead of a static string
value.

The struct (conf.h) changed from otelc_kv/attr_len to a list of
flt_otel_conf_sample entries.  The parser (parser.c) calls
flt_otel_parse_cfg_sample() with n=1 per attr keyword.  At runtime
(event.c) each attribute is evaluated via flt_otel_sample_eval() and added
via flt_otel_sample_add_kv() to a bare flt_otel_scope_data_kv, which is
passed to logger->log_span().

Updated documentation, debug macro and test configurations.