Handling and validation of metadata in the requests?

[kwilczynski]

Related to TritonDataCenter/triton-go#151.

We need to make sure that we can handle and validate the following scenarios when handling metadata:

• No matadata (not added at all)
• No metadata (as null)
• Empty object (not as null)
• Nested objects (another key-value pair/object inside)

We are assuming that metadata almost always is flat key-value map, but that might not always be the case (see the linked issue above) - and we won't be able to correctly parse JSON with a nested object into a type (struct) on the Go side, which could result in a panic being thrown.