Net6: runtime error 'Synchronous operations are disallowed'

[SpiridonovAlexMoeDelo]

On each call to /metrics I get next runtime error in application log:

Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead. (System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.)

We need to use async dispose to avoid it.

Suggested changes to fix it: https://github.com/AppMetrics/AppMetrics/pull/503/files#r1212777476