add support for multiple log formats (#517)

this commit adds the tracing-subscriber crate and use its formatters to
support multiple log formats.

More details in
https://github.com/postgresml/pgcat/issues/464#issuecomment-1641430299

Signed-off-by: Sebastian Webber <sebastian@swebber.me>

src/cmd_args.rs

@@ -1,17 +1,27 @@
-use clap::Parser;
-use log::LevelFilter;
+use clap::{Parser, ValueEnum};
+use tracing::Level;
 
 /// PgCat: Nextgen PostgreSQL Pooler
 #[derive(Parser, Debug)]
 #[command(author, version, about, long_about = None)]
-pub(crate) struct Args {
+pub struct Args {
     #[arg(default_value_t = String::from("pgcat.toml"), env)]
     pub config_file: String,
 
-    #[arg(short, long, default_value_t = LevelFilter::Info, env)]
-    pub log_level: log::LevelFilter,
+    #[arg(short, long, default_value_t = tracing::Level::INFO, env)]
+    pub log_level: Level,
+
+    #[clap(short='F', long, value_enum, default_value_t=LogFormat::Text, env)]
+    pub log_format: LogFormat,
 }
 
-pub(crate) fn parse() -> Args {
+pub fn parse() -> Args {
     return Args::parse();
 }
+
+#[derive(ValueEnum, Clone, Debug)]
+pub enum LogFormat {
+    Text,
+    Structured,
+    Debug,
+}