feat: Flush async counter

[bruno-garcia]

No description provided.

[codecov-io]

Codecov Report

Merging #214 into master will increase coverage by 0.16%.
The diff coverage is 79.68%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   88.48%   88.65%   +0.16%     
==========================================
  Files          81       81              
  Lines        1963     2018      +55     
==========================================
+ Hits         1737     1789      +52     
- Misses        226      229       +3

Impacted Files	Coverage Δ
src/Sentry/Extensibility/HubAdapter.cs	95.45% <0%> (-4.55%)	⬇️
src/Sentry/Internal/Hub.cs	61.19% <0%> (-7.14%)	⬇️
src/Sentry/SentryClient.cs	95.06% <0%> (-1.19%)	⬇️
src/Sentry/Extensibility/DisabledHub.cs	100% <100%> (ø)	⬆️
src/Sentry/SentrySdk.cs	100% <100%> (ø)	⬆️
src/Sentry/Internal/OptionalHub.cs	76.19% <100%> (+1.19%)	⬆️
src/Sentry/Internal/BackgroundWorker.cs	95.9% <92.3%> (-2.84%)	⬇️
...Sentry/Extensibility/DiagnosticLoggerExtensions.cs	84% <0%> (+40%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d17229...0b407f5. Read the comment docs.

[HazAT]

One nitpick