Add suppress_instrumentation flag in context for Metrics

[hectorhdzg]

Similar to logic in SpanProcessors, this need to be done in Metrics to avoid duplicated telemetry when using Http ext or other packages relying on this

[lzchen]

Curious as to how we are going to handle the case of when dependency calls being made at the same time export is being called. With this logic, calls during when suppress_instrumentation flag is true (during export) will not be tracked.

[lzchen]

EDIT: It seems like context is local per thread so that use case won't be an issue.