MINOR: otel: added log-record signal support

MINOR: otel: added log-record signal support

Added "log-record" as the third OpenTelemetry signal alongside traces
(span) and metrics (instrument).  This includes the
flt_otel_conf_log_record structure definition, parser keyword defines,
the otel-scope section parser with optional "id", "event", "span", and
"attr" keywords followed by sample fetch expressions or a log-format
string, init/free lifecycle, scope list wiring, log-format evaluation
in flt_otel_scope_run_instrument_record(), a test configuration example,
log-record span reference validation in flt_otel_check(), and logger
handle creation, startup, and teardown in the filter lifecycle.