Renaming Meter to Accumulator in Metrics SDK context

[AzfaarQureshi]

Description

This PR addresses Issue open-telemetry#1342 and issue open-telemetry#1307 by renaming the Meter class in the metrics sdk to Accumulator

Summary of changes

1. rename Meter class to Accumulator
2. rename all meter = meter_provider.get_meter() to accumulator = meter_provider.get_meter() for consistency

Rationale behind changes

1. rename Meter class to Accumulator

• The Meter Class does a lot of the same things as the Accumulator in Go

Similarities	Meter class	Accumulator struct
collects all metrics	def collect	func collect
helper func to collect from sync instruments	def _collect_metrics	func collectSyncInstruments
helper func to collect from async instruments	def _collect_observers	func observeAsyncInstruments
record batch metric events	def record_batch	func RecordBatch
register sync instruments	create_counter, create_updowncounter etc	func NewSyncInstrument
register async instruments	register_sumobserver, register_updownsumobserver etc	func NewAsyncInstrument

• The Meter class does all of what is defined for theAccumulator in the spec
  • extends the Meter interface
  • adds collection methods for Synchronous and Asynchronous instruments (which are metrics and observers respectively in Python)
  • registers records to concurrency update and aggregate records of metric data

2. rename all meter = meter_provider.get_meter() to accumulator = meter_provider.get_meter() for consistency

• feels awkward to have an instance of a meter since a meter is an interface
• now that the metrics sdk meter_provider returns an accumulator, the usage in various tests and examples should reflect what is actually being returned

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Just made sure all the tests are passing post name changes.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[alolita]

lgtm