agent: Only show ttrpc logs for trace log level

Only display the `ttrpc` crate log output when full logging
(trace level) is enabled.

This is a slight abuse of log levels but provides developers and testers
what they need whilst also keeping the logs relatively quiet for the
default info log level (the `ttrpc` crate logging is a bit "chatty").

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

src/agent/Cargo.lock

@@ -375,6 +375,7 @@ dependencies = [
  "cgroups",
  "lazy_static",
  "libc",
+ "log",
  "logging",
  "netlink",
  "nix 0.17.0",

src/agent/Cargo.toml

@@ -30,6 +30,7 @@ slog-scope = "4.1.2"
 
 # Redirect ttrpc log calls
 slog-stdlog = "4.0.0"
+log = "0.4.11"
 
 # for testing
 tempfile = "3.1.0"

src/agent/src/main.rs

@@ -198,8 +198,12 @@ fn main() -> Result<()> {
     // which is required to satisfy the the lifetime constraints of the auto-generated gRPC code.
     let _guard = slog_scope::set_global_logger(logger.new(o!("subsystem" => "rpc")));
 
-    // Redirect ttrpc log calls to slog
-    let _log_guard = slog_stdlog::init()?;
+    let mut _log_guard: Result<(), log::SetLoggerError> = Ok(());
+
+    if config.log_level == slog::Level::Trace {
+        // Redirect ttrpc log calls to slog iff full debug requested
+        _log_guard = Ok(slog_stdlog::init().map_err(|e| e)?);
+    }
 
     start_sandbox(&logger, &config, init_mode)?;