Support richer playground configuration

[eyurtsev]

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Discussed in https://github.com/langchain-ai/langserve/discussions/547

^{Originally posted by SoulEvill March 19, 2024} I created a sequence of runnable and would like to test it out in the playground. the endpoint is up but when I click on start - maybe because of complicated chain that I have it was returning repeated result. I looked in the server log and realized the playground 'start' button was calling the stream_log - i was wondering if this behavior could be config - I would like to change it from stream_log to invoke.

sample log information from server when click on start button from playground: "127.0.0.1:49397 - "POST /xxxx/stream_log HTTP/1.1" 404 Not Found"

any help would be really appreciated! Thanks!