Metadata should have a nicely typed `put` method

[ydolzhenko]

metadata setter of ResponseCreateParams.builder()accepts Metadata object, which, in turn may contain ```
numeric fields:

        val metadata = Metadata.builder()
            .putAdditionalProperty("someNumericId", JsonNumber.of(123))
            .build();

Response with such metadata instance fails though:

ResponseCreateParams.builder().metadata(metadata).build()....

showing error:

com.openai.errors.BadRequestException: 400: OpenAIError{additionalProperties={error={message=Invalid type for 'metadata.someNumericId': expected a string, but got an integer instead., type=invalid_request_error, param=metadata.someNumericId, code=invalid_type}}}

String as a value works fine though.

[IvanLuchkin]

The SDK does not do automatic conversion.

[TomerAberbach]

Yeah, this is sort of a bug in the SDK I would say.

It is true that the API only accepts strings, but for most other classes in the SDK we accept nicely typed parameters somehow and then have putAdditionalProperty for undocumented values.

So I think we need to update Metadata to have something like putProperty(String, String). Thanks for flagging :)