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Network flow logs are a common feature of VPNs. Due to the nature of a shared exit node, it is of great interest to a network analyst, which TCP connections are getting routed through the tunnel, who is initiating them, for long do they last and how much traffic is sent across them. With this PR, the Firezone Gateway gains the ability of detecting the TCP and UDP flows that are being routed through it. The information we want to attach to these flows is spread out over several layers of the packet handling code. To simplify the implementation and not complicate the APIs unnecessarily, we chose to rely on TLS (thread-local storage) for gathering all the necessary data as a packet gets passed through the various layers. When using a const initializer, the overhead of a TLS variable over an actual local variable is basically zero. The entire routing state of the Gateway is also never sent across any threads, making TLS variables a particularly good choice for this problem. In its MVP form, the detected flows are only emitted on stdout and also that only if `flow_logs=trace` is set using `RUST_LOG`. Early adopters of this feature are encouraged to enable these logs as described and then ingest the Gateway's logs into the SIEM of their choice for further analysis. Related: #8353