OTLP Gauge datapoint timestamp should be time at sampling

[lenin-jaganathan]

micrometer/implementations/micrometer-registry-otlp/src/main/java/io/micrometer/registry/otlp/OtlpMetricConverter.java

Line 102 in c727c3f

.setTimeUnixNano(getTimeUnixNano())

As per OTLP Spec, the time_unix_nano of Gauge datapoint should be "A timestamp when the value was sampled". For cumulative implementation, we already set the time to the current instant. The delta implementation should also follow the same.