Add metrics exporter requirements to the metrics SDK spec

[reyang]

Follow up #1837.

Changes

• Added the metrics exporter type
• Added more information to the In-memory exporter

[jmacd]

Discussed in the SIG call, and not mentioned in the comments:

The OTel-Go prototype metrics Reader interface, associated with pull exporters and push exporters alike, accepts an ExportKindSelector that allows each exporter to choose the export strategy it desires.

The prototype's basic processor, here, keeps sufficient state during the collect-and-process code path to support both delta and cumulative exporters at once, provided it was given the proper configuration up front.

The "Reader" API (presently named CheckpointSet) accepts the ExportKindSelector here in its ForEach() method for iterating over the results of Collect() in both Push and Pull code paths.

The "configuration up front" is provided via the basic Processor's constructor. Each Exporter is also an ExportKindSelector, meaning the exporter will be consulted once per *metric.Descriptor as to the choice of export strategy. ExportKind is a bit set, allowing multiple exporters to select both export strategies when desired.

// ExportKind indicates the kind of data exported by an exporter.
// These bits may be OR-d together when multiple exporters are in use.
type ExportKind int

const (
	// CumulativeExportKind indicates that an Exporter expects a
	// Cumulative Aggregation.
	CumulativeExportKind ExportKind = 1

	// DeltaExportKind indicates that an Exporter expects a
	// Delta Aggregation.
	DeltaExportKind ExportKind = 2
)

[jmacd]

Thanks @reyang.