feat(om2): exemplar timestamp MAY to SHOULD (#2775)

Make the exemplar timestamp a SHOULD in preparation for complex types
having multiple exemplars at a time. We'll explain why having timestamp
for exemplars of complex types is a good idea there.

Signed-off-by: György Krajcsovits <[email protected]>

Author: krajorama

docs/specs/om/open_metrics_spec_2_0.md

@@ -108,7 +108,7 @@ Each MetricPoint consists of a set of values, depending on the MetricFamily type
 
 Exemplars are references to data outside of the MetricSet. A common use case are IDs of program traces.
 
-Exemplars MUST consist of a LabelSet and a value, and MAY have a timestamp. They MAY each be different from the MetricPoints' LabelSet and timestamp.
+Exemplars MUST consist of a LabelSet and a value, and SHOULD have a timestamp. They MAY each be different from the MetricPoints' LabelSet and timestamp.
 
 The combined length of the label names and values of an Exemplar's LabelSet MUST NOT exceed 128 UTF-8 character code points. Other characters in the text rendering of an exemplar such as `",=` are not included in this limit for implementation simplicity and for consistency between the text and proto formats.