Metric instruments: default-enabled clarification

[jmacd]

What are you trying to achieve?

The review thread for #2404 points to lack of clarity around the statement for what happens when Views are configured and none matches the instrument. See the comment thread here: #2404 (comment)

The original text leaves a lot of room for interpretation, perhaps too much.

   * If the Instrument could not match with any of the registered `View`(s), the
    SDK SHOULD provide a default behavior. The SDK SHOULD also provide a way for
    the user to turn off the default behavior via MeterProvider (which means the
    Instrument will be ignored when there is no match). Individual
    implementations can decide what the default behavior is, and how to turn the
    default behavior off.

Additional context.

Discussed in the 3/15 Spec SIG. One proposal appears to be favored, which would require no new parameters. If instruments are enabled when no View matches, then users can always fill in a match-all expression to disable instruments using Views to achieve the effect of instruments being disabled-by-default.