Add optional "label" arg to promptlayer.track.prompt

[jzone3]

Tested it with code below:

import promptlayer

import os
from dotenv import load_dotenv

# Load environment variables from a .env file
load_dotenv()

promptlayer.api_key = os.getenv("PROMPTLAYER_API_KEY")
openai = promptlayer.openai
openai.api_key = os.getenv("OPENAI_API_KEY")

template= promptlayer.prompts.get("tell_a_story")
input_vars = {"summary": "Hello", "original": "Goodbye"}
prompt = template['template'].format(**input_vars)

resp, pl_id = openai.Completion.create(model="gpt-3.5-turbo-instruct", prompt=prompt, return_pl_id=True)

track = promptlayer.track.prompt(
  request_id=pl_id, 
  prompt_name="tell_a_story",
  prompt_input_variables=input_vars,
  label="second",
  version=1
)
print(track)