Run with:

Lexer.g4

lexer grammar Lexer;
fragment WS: [\t ]+;
fragment NL: WS? ('\r'* '\n')+;
COLON_ATTRIBUTES: ('KeyId' |
                   'first key' |
                   'key' |
                   'key (interesting)' |
                   'key(# of 1second)' |
                   'key.subkey')+ -> mode(COLON_ATTRIBUTES_MODE);

mode COLON_ATTRIBUTES_MODE; //key: value is whatever might come before the newline
COLON_ATTRIBUTES_INTERMEDIATE: (' '|'\t')* ':' (' '|'\t')* -> channel(HIDDEN);
COLON_ATTRIBUTES_DATA: ~[\r\n]+;
COLON_ATTRIBUTES_END: NL -> channel(HIDDEN), mode(DEFAULT_MODE);

Listener.py

from antlr4 import *
if __name__ is not None and "." in __name__:
    from .Parser import Parser
else:
    from Parser import Parser

# This class defines a complete listener for a parse tree produced by Parser.
class Listener(ParseTreeListener):

    # Enter a parse tree produced by Parser#whole_log.
    def enterWhole_log(self, ctx:Parser.Whole_logContext):
        pass

    # Exit a parse tree produced by Parser#whole_log.
    def exitWhole_log(self, ctx:Parser.Whole_logContext):
        pass


    # Enter a parse tree produced by Parser#colon_attributes.
    def enterColon_attributes(self, ctx:Parser.Colon_attributesContext):
        print([c.getText() for c in ctx.getChildren()])
        pass

    # Exit a parse tree produced by Parser#colon_attributes.
    def exitColon_attributes(self, ctx:Parser.Colon_attributesContext):
        pass



del Parser

Parser.g4

parser grammar Parser;

options {
  tokenVocab=Lexer;
}

whole_log: colon_attributes+;
colon_attributes: COLON_ATTRIBUTES COLON_ATTRIBUTES_INTERMEDIATE COLON_ATTRIBUTES_DATA COLON_ATTRIBUTES_END;

test.py

"""
"C:\Program Files\JetBrains\PyCharm Community Edition 2020.1\jbr\bin\java.exe" -Xmx500M -cp "C:\Users\nmz787\Downloads\antlr-4.8-complete.jar" org.antlr.v4.Tool Lexer.g4 -Dlanguage=Python3
"C:\Program Files\JetBrains\PyCharm Community Edition 2020.1\jbr\bin\java.exe" -Xmx500M -cp "C:\Users\nmz787\Downloads\antlr-4.8-complete.jar" org.antlr.v4.Tool Parser.g4 -Dlanguage=Python3
"""

from antlr4 import CommonTokenStream, ParseTreeWalker, InputStream
inp = InputStream("""key: value
""")

from Lexer import Lexer
from Parser import Parser
from Listener import Listener

lexer = Lexer(inp)
stream = CommonTokenStream(lexer)
parser = Parser(stream)
printer = Listener()
# parser.removeErrorListeners()
# lexer.removeErrorListeners()
tree = parser.whole_log()
walker = ParseTreeWalker()
walker.walk(printer, tree)