feat(om2): expand Number description with float64 and NaN details (#2928)

- Add IEEE 754-2008 double-precision reference for float64
- Expand NaN usage to cover all undefined/indeterminate mathematical results


Coded with Claude Sonnet 4.6.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

Author: krajorama

docs/specs/om/open_metrics_spec_2_0.md

@@ -130,7 +130,9 @@ Metric values in OpenMetrics MUST be either Number or CompositeValue.
 
 ##### Number
 
-Number value MUST be either floating point or integer. Note that ingestors of the format MAY only support float64. The non-real values NaN, +Inf and -Inf MUST be supported. NaN value MUST NOT be considered a missing value, but it MAY be used to signal a division by zero.
+Number value MUST be either floating point or integer.
+
+Note that ingestors of the format MAY only support float64, for example Go's `float64` which is an IEEE 754-2008 double-precision (binary64) floating-point number with approximately 15–17 significant decimal digits of precision. The non-real values NaN, +Inf and -Inf MUST be supported. NaN value MUST NOT be considered a missing value, but it MAY be used to signal a division by zero or any other mathematical operation that yields an undefined or indeterminate result.
 
 Booleans MUST be represented as a Number value where `1` is true and `0` is false.