[dashboard] fix peek parse message error

[yittg]

Fixes #4917

Motivation

dashboard peek message api raise exception when format response cause treat all message as a JSON string

Modifications

• format as JSON only if message is a JSON,
• otherwise if message is printable, return the original message,
• otherwise print hex like command with --hex option.

[yittg]

cc @sijie @rshermanTHG

[rshermanTHG]

From a functionality point of view looks good to me. Can't really comment on how Pythonic the code is as I only hack Python.

[yittg]

take a closer look about the api implementation, found that this is a wrong deserialize implementation, it didn't distinguish the batch mode or not.

sorry for that, just hold it.

[yittg]

For now, we can not parse batch messages payload without metadata pb struct except those with size of 1. So in this PR, only parse rightly for normal and 1-batch message, and display the size only of other batch messages.

It can be resolved in another PR for those batch messages if need.

@sijie what do you think?

[sijie]

@yittg sgtm

[yittg]

@sijie yeah, let's do it, PTAL. by the way, here's some screenshots:

[yittg]

run cpp tests
run integration tests

[sijie]

@tuteng can you please help review this?

[tuteng]

lgtm