I'm not exactly an expert with LibCST so maybe the function can be optimized further, but here is an example with minimal code for both LibCST and pfst to add a keyword argument with a comment to all logger.info() calls which don't already have the specific keyword argument correlation_id.

LibCST function:

from libcst import *
from libcst.matchers import *

def inject_logging_metadata(src: str) -> str:
    module = parse_module(src)

    new_arg = (parse_module('f(correlation_id=CID  # blah\n)')
        .body[0].body[0].value.args[0])

    class AddArg(CSTTransformer):
        def leave_Call(self, _, node):
            if matches(node.func, Attribute(Name('logger'), Name('info'))):
                if not any(a.keyword and a.keyword.value == 'correlation_id'
                        for a in node.args):
                    return node.with_changes(args=[*node.args, new_arg])
            return node

    module = module.visit(AddArg())

    return module.code

pfst function:

from fst import *
from fst.match import *

def inject_logging_metadata(src: str) -> str:
    module = FST(src)

    for m in module.search(MCall(
        func=MAttribute('logger', 'info'),
        keywords=MNOT([MQSTAR, Mkeyword('correlation_id'), MQSTAR]),
    )):
        m.matched.append('correlation_id=CID  # blah', trivia=())

    return module.src

Input source:

logger.info('Hello world...')  # hey
logger.info('Already have id', correlation_id=other_cid)  # ho
logger.info()  # its off to work we go

class cls:
    def method(self, thing, extra):
        (logger) . info(  # start
            f'a {thing}',  # this is fine
            extra=extra,  # also this
        )  # end

LibCST output:

logger.info('Hello world...', correlation_id=CID  # blah
)  # hey
logger.info('Already have id', correlation_id=other_cid)  # ho
logger.info(correlation_id=CID  # blah
)  # its off to work we go

class cls:
    def method(self, thing, extra):
        (logger) . info(  # start
            f'a {thing}',  # this is fine
            extra=extra,  # also this
        correlation_id=CID  # blah
        )  # end

pfst output:

logger.info('Hello world...', correlation_id=CID  # blah
)  # hey
logger.info('Already have id', correlation_id=other_cid)  # ho
logger.info(correlation_id=CID  # blah
)  # its off to work we go

class cls:
    def method(self, thing, extra):
        (logger) . info(  # start
            f'a {thing}',  # this is fine
            extra=extra,  # also this
            correlation_id=CID  # blah
        )  # end

If you want LibCST to align the argument its significantly more code, but that can be left for a formatter after the file processing.