[http] Measure per-connection and per-request memory usage

It would be good to have some benchmarks that measure memory usage in the following scenarios:

• H1 and H2 request after proxying request headers to upstream
• H1 and H2 request after proxying response headers to client
• Idle H1 client connection using raw sockets and TLS sockets
• Idle H2 client connection with empty/full compression context

Implementing strict tests like test/integration/stats_integration_test.cc for codec memory usage may not be feasible due to the codecs having a higher rate of change. Use of a mechanism like the one proposed in https://github.com/envoyproxy/envoy/issues/11266 to track these performance metrics and flamegraphs of the heapz output from CI would allow us to detect and track down improvements/regressions.

@oschaaf