ExecutionService: check throttling when an operation is scheduled

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

Description

This MR support deprioritizing a to-be-scheduled ExecutionRequest if the client has over-consumed the computing resources above a predefined threshold in metering-servce

This feature is optional so backward compatible with existing BuildGrid versions.

Changes proposed in this merge request:

• Add parser for MeteringServiceClient
• Call get_throttling before an ExecutionRequest is fulfilled
• If a request should be throttled, the current logic is to depriorize it -- by changing the priority to 1
  • Note that the default priority is 0.

Minor

Introducing the new dependency seems to make simple pip-compile impossible so I also upgraded all dependencies.

• Unpin aiohttp
• Re-generate requirements.txt
• Python GRPC doesn't accept uppercase header keys, so they are lowercased.
• Rename execution_instance.py to test_execution_instance.py for pytest auto discovery.