event: fix DispatcherImplTest::InitializeStats flake (#6619)
event: fix DispatcherImplTest::InitializeStats flake (#6619)

Fix DispatcherImplTest::InitializeStats flake due to dispatcher lifetime exceeding mock stats scope lifetime.

Risk Level: low
Testing: bazel test //test/common/event:dispatcher_impl_test --runs_per_test=1000 (with ASAN enabled)
Docs Changes: n/a
Release Notes: n/a

Fixes #6611

Signed-off-by: Dan Rosen <cd79e6963e0fec0b9501139f6e63499ce4af39a7@google.com>

dnoe/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy,eklitzke/envoy,lyft/envoy,jrajahalme/envoy,lyft/envoy,istio/envoy,jrajahalme/envoy,jrajahalme/envoy,lizan/envoy,dnoe/envoy,eklitzke/envoy,envoyproxy/envoy,istio/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,lyft/envoy,istio/envoy,lyft/envoy,lizan/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,dnoe/envoy,eklitzke/envoy,lyft/envoy,dnoe/envoy,dnoe/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy-wasm,istio/envoy,eklitzke/envoy,istio/envoy,lyft/envoy,eklitzke/envoy,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy