Add dimension, metric and namespace validation

[markkuhn]

Related to:

• https://github.com/awslabs/aws-embedded-metrics-node/pull/135
• https://github.com/awslabs/aws-embedded-metrics-java/pull/119
• https://github.com/awslabs/aws-embedded-metrics-python/pull/91

Description of changes: Added validation for dimension, metric and namespace

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