Feature Request: annotation for method timing metrics

[jkwatson]

Is your feature request related to a problem? Please describe.
Not all methods are appropriate for having spans generated for them (especially very high volume methods in an otherwise relatively low-volume request path). It would be very useful to be able to generate method-timing metrics for methods like this.

Describe the solution you'd like
Analogous to @WithSpan, something like @Timed at the method level.

Describe alternatives you've considered
Manually creating metrics would of course work, but is much more boilerplate code, especially in the absence of a top-level Timer instrument.

Open Questions
Does each of these generate a new Instrument? Or use a common instrument, and add an attribute for the method name?

[trask]

Closing in favor of #7030