Fix Zipkin AddEvent() timestamp

[lalitb]

Changes

A small change to record the Zipkin event timestamp in microseconds, as in the specs:
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk_exporters/zipkin.md#unit-of-time
Zipkin times like timestamp, duration and annotation.timestamp MUST be reported in microseconds as whole numbers. For example, duration of 1234 nanoseconds will be represented as 1 microsecond.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #896 (9686df5) into main (124087f) will not change coverage.
The diff coverage is n/a.

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##             main     #896   +/-   ##
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  Coverage   95.40%   95.40%           
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  Files         158      158           
  Lines        6717     6717           
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  Hits         6408     6408           
  Misses        309      309