feat(gateway): add option for outputting logs as JSON (#10620)

To enable customers to ingest flow logs (#8353) into various SIEMS,
outputting structured logs is crucial.

rust/gateway/src/main.rs

@@ -22,7 +22,7 @@ use phoenix_channel::get_user_agent;
 
 use phoenix_channel::PhoenixChannel;
 use secrecy::{Secret, SecretString};
-use std::{collections::BTreeSet, path::Path};
+use std::{collections::BTreeSet, fmt, path::Path};
 use std::{path::PathBuf, process::ExitCode};
 use std::{sync::Arc, time::Duration};
 use tracing_subscriber::layer;
@@ -91,8 +91,14 @@ fn has_necessary_permissions() -> bool {
 }
 
 async fn try_main(cli: Cli, telemetry: &mut Telemetry) -> Result<()> {
-    firezone_logging::setup_global_subscriber(layer::Identity::default())
-        .context("Failed to set up logging")?;
+    firezone_logging::setup_global_subscriber(
+        layer::Identity::default(),
+        match cli.log_format {
+            LogFormat::Json => true,
+            LogFormat::Human => false,
+        },
+    )
+    .context("Failed to set up logging")?;
 
     tracing::info!(
         arch = std::env::consts::ARCH,
@@ -306,6 +312,9 @@ struct Cli {
     #[arg(short = 'i', long, env = "FIREZONE_ID")]
     firezone_id: Option<String>,
 
+    #[arg(long, env = "FIREZONE_LOG_FORMAT", default_value_t = LogFormat::Human)]
+    log_format: LogFormat,
+
     /// Where to export metrics to.
     ///
     /// This configuration option is private API and has no stability guarantees.
@@ -336,6 +345,21 @@ struct Cli {
     no_inc_buf: bool,
 }
 
+#[derive(Debug, Clone, Copy, clap::ValueEnum)]
+enum LogFormat {
+    Json,
+    Human,
+}
+
+impl fmt::Display for LogFormat {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            LogFormat::Json => write!(f, "json"),
+            LogFormat::Human => write!(f, "human"),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy, clap::ValueEnum)]
 enum MetricsExporter {
     Stdout,