MINOR: otel: added metrics instrument support

MINOR: otel: added metrics instrument support

Added the "instrument" keyword to otel-scope sections for recording metric
measurements alongside traces.

Introduced flt_otel_conf_instrument holding instrument type, description,
unit, sample expressions, and optional key-value attributes.  The
supported synchronous integer-precision instrument types were counters,
histograms, up-down counters, and gauges.

Instruments followed a two-form design: a "create" form defined a new
instrument with its type and value expression, while an "update" form
recorded measurements against an existing instrument with per-scope
attributes.

Instrument creation was performed lazily at first use with HA_ATOMIC_CAS
to guarantee thread-safe one-time initialization.  The configuration
check phase validated that every update-form had a matching create-form
definition and that create-form names were unique across all scopes.

The meter lifecycle was integrated into filter init and deinit, starting
the meter alongside the tracer and shutting it down during cleanup.