Add support of just doing metrics streaming with client api

[YuanTingHsieh]

Add support when people just want to use metrics streaming (i.e. Metric Pipe) of client api.

Description

• Make "task_pipe" non-required
• Make TASK_EXCHANGE section optional for client api config

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Quick tests passed locally by running ./runtest.sh.
• [ ] In-line docstrings updated.
• [ ] Documentation updated.

[YuanTingHsieh]

/build

[YuanTingHsieh]

/build

[chesterxgchen]

/build

[YuanTingHsieh]

/build

[YuanTingHsieh]

/build