access log: speed up the access log formatter (#3159)
access log: speed up the access log formatter (#3159)

This change reduces the CPU usage of the access log formatter ~10% by using fmt::FormatInt in place of the much slower std::to_string.

Benchmark results:
```
-------------------------------------------------------------
Benchmark                      Time           CPU Iterations
-------------------------------------------------------------
Before:
BM_AccessLogFormatter       5401 ns       5401 ns     129597

After:
BM_AccessLogFormatter       4788 ns       4788 ns     146003
```

Risk Level: Medium
Testing: unit tests
Docs Changes: N/A
Release Notes: N/A
Issues: #2879

Signed-off-by: Brian Pane <5720439aa8fe33541c355982794f1a4cb6b27628@pinterest.com>
envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy-wasm,istio/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy,sebrandon1/envoy,lizan/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm,istio/envoy,lyft/envoy,eklitzke/envoy,dnoe/envoy,eklitzke/envoy,envoyproxy/envoy,sebrandon1/envoy,istio/envoy,lizan/envoy,sebrandon1/envoy,jrajahalme/envoy,jrajahalme/envoy,jrajahalme/envoy,lyft/envoy,envoyproxy/envoy-wasm,dnoe/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy,dnoe/envoy,envoyproxy/envoy,eklitzke/envoy,eklitzke/envoy,istio/envoy,lizan/envoy,jrajahalme/envoy,sebrandon1/envoy,dnoe/envoy,jrajahalme/envoy,envoyproxy/envoy,lizan/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,lyft/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,lyft/envoy,sebrandon1/envoy,envoyproxy/envoy,envoyproxy/envoy,istio/envoy