Metric - concurrency test improvements

[cijothomas]

Current tests are very limited: https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/test/OpenTelemetry.Tests/Metrics/MetricAPITest.cs#L241

1. Need to test the special case of "no tags".
2. Need to add test where different tags are updated by diff. threads. (To catch these https://github.com/open-telemetry/opentelemetry-dotnet/pull/2339#discussion_r711239998)
3. Need to add test where same tags are updated by diff. threads. (already covered for single dimension. Need to modify for more dimensions as there are special ThreadLocal optimizations for upto N.)

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