fix: Fix timestamp drift due to using instanceof typeguard

[cindy-peng]

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Fixes #802 🦕 This is to fix timestamp drift issue which may cause timestamp inaccuracy while using OpenTelemetry and view trace timespans. Since timestamp within metadata is defined as a union-types including null and undefined, the type guard is not necessary here.

[daniel-sanche]

Can you add any tests around this? Try to make one that would fail before the change, but passes after

[cindy-peng]

Can you add any tests around this? Try to make one that would fail before the change, but passes after

Good suggestion. Will do!