Added support for more than 100 metrics

[yaozhaoy]

The EMF spec limits the max number of metrics in one log event to 100. The java library needs to follow the spec

Metrics— An array of MetricDefinition objects. This array MUST NOT contain more than 100 MetricDefinition objects.

[janimationd]

Hi, now that this is resolved, can you please update the Readme.md with the new limit information? https://github.com/awslabs/aws-embedded-metrics-java/blob/master/README.md?plain=1#L80