Only build metadata payload NDJSON string once

[watson]

Only stringify the metadata object once and reuse the generated string for all future metadata events being sent.

[alemagio]

Hi, i would like to work on this.
Tell me if i understood the problem:

• In the _encode function of the Client.prototype we serialize the entire output
• what we want is to save the serialized metadata and reuse it instead of doing the serialization every time.
  Correct?

[Qard]

Yeah, what I would do is store the encoded version on the client and just change this line to write the already encoded content rather than redoing the encoding every time. You'll want to make sure the pre-encoded metadata is rebuilt any time the config changes though--the end of Client.prototype.config is probably a good place to generate and store the pre-encoded metadata header.