def metric():
while True:<EOL><INDENT>yield <NUM_LIT:0><EOL><DEDENT>
A metric is a number stream that Relay polls every so often.
    Relay implicitly modifies this number stream when it calls the
    warmer() and cooler() functions.
    Relay attempts to minimize the difference between the current metric
    value and your current target value.

    This function could, for instance, yield the current temperature

    This function should be a generator.