Add temporality option to readers

[MrAlias]

Measurement temporality is defined at the reader level in the specification:

To construct a MetricReader when setting up an SDK, the caller SHOULD provide at least the following:
[...]

• The default output temporality (optional), a function of instrument kind. If not configured, the Cumulative temporality SHOULD be used.

The manual and periodic readers need to accept an option to configure this.

[MrAlias]

Possible implementation:

package metric

// Temporality is a measurement intervals relationship to time.
type Temporality uint8

const (
	// CumulativeTemporality defines a measurement interval that continues to
	// expand forward in time from a starting point. New measurements are
	// added to all previous measurements since a start time.
	CumulativeTemporality Temporality = iota

	// DeltaTemporality defines a measurement interval that resets each cycle.
	// Measurements from one cycle are recorded independently, measurements
	// from other cycles do not affect them.
	DeltaTemporality
)

// WithTemporality uses the selector to determine the Temporality measurements
// from instrument should be recorded with.
func WithTemporality(selector func(instrument view.Instrument) Temporality) PeriodicReaderOption {
	/* ... */
}

// Add a similar option for a manual reader.

[MrAlias]

Blocked on the view.Instrument or equivalent type being added.

[MadVikingGod]

I think there will also need to be a way for the Meter to request this information from a reader, so it can use it when creating the instrument. So we should probably expose something like this in the Reader interface.

[Aneurysm9]

We should be careful how we define constants to represent these options and consider keeping them in line with the OTLP definitions, which would be:

const (
  undefinedTemporality Temporality = iota
  DeltaTemporality
  CumulativeTemporality
)

We would probably want to leave undefinedTemporality unexported as it should never be explicitly provided.

[MrAlias]

Closed #2949