[Metrics] Metrics SDK export pipeline tracking issue

[legendecas]

As controllers are responsible for coordinating the Accumulator, Processor, and Exporter components in an export pipeline, it is important to continue the work of #1017 before we have a spec of the solid definition of such pipeline.

Another important change with the latest spec of metrics sdk is the temporality of aggregators and how processors playing the role in accumulating aggregator snapshots to reduce the penalties on delta exporting pipelines.

• Rename Batcher to Processor (ungrouped/without cumulation): Metrics updates #1700;
• Processor processing phase interface feat: processor export phases interface #1717;
• Aggregator snapshots;
• Basic processor supporting both CumulativeExportKindSelector and StatelessExportKindSelector;
• Reducing processor;
• Controller interface;

[legendecas]

Closing as spec updates. This is no longer valid.