stats: hitting flushing while already flushing assertion

[junr03]

SIGABRT: Abort program
        at tgkill(/system/lib64/libc.so:440272)
        at abort(/system/lib64/libc.so:122644)
        at Envoy::Stats::ThreadLocalStoreImpl::mergeHistograms(std::__ndk1::function<void ()>)(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:15809952)
        at Envoy::Server::InstanceImpl::flushStats()(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:14918788)
        at Envoy::Event::DispatcherImpl::runPostCallbacks()(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:15486116)
        at 0x7e65064ce4(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:18566372)
        at event_base_loop(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:18549540)
        at Envoy::Server::InstanceImpl::run()(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:14924404)
        at Envoy::MainCommonBase::run()(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:14104948)
        at Envoy::Engine::run(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >)(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:13849616)
        at void* std::__ndk1::__thread_proxy<std::__ndk1::tuple<std::__ndk1::unique_ptr<std::__ndk1::__thread_struct, std::__ndk1::default_delete<std::__ndk1::__thread_struct> >, envoy_status_t (Envoy::Engine::*)(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >), Envoy::Engine*, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> > > >(void*)(/data/app/me.lyft.android.beta-Kvj80d62eAicoIFuNGT_HA==/lib/arm64/libenvoy_jni.so:13923388)
        at 0x7e9d042150(/system/lib64/libc.so:426320)
        at 0x7e9cff9668(/system/lib64/libc.so:128616)
        at 0x0(unknown)

Due to this being a sigabort it is clear to see that we are hitting the following assertion: https://github.com/envoyproxy/envoy/blob/609458bbd178100f956ee8587c1092abfb9a99f2/source/common/stats/thread_local_store.cc#L167

This started happening since we enabled #717. It makes sense as the app might have the opportunity to trigger a subsequent flush that causes merging before the previous merge finishes.

[junr03]

closing this in favor of #754, as the crash itself won't happen now that the lifecycle flushing is disabled.