Metrics SDK multi-threaded testing

[cijothomas]

This was discussed in today's community meeting:

1. We lack unit testing for Metrics.
2. We have some module level tests https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-sdk/src/metrics/mod.rs that also tests basic scenarios only

We need some tests to orchestrate more complex usage pattern (esp. involving multiple threads updating instruments), and validate that the aggregation is mathematically correct.
Here is an example of such test from OTel .NET (https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/test/OpenTelemetry.Tests/Metrics/MetricApiTestsBase.cs#L1460-L1462)

@bIgBV has agreed to make this happen.