Move observability config to separate config section

This renames:

allow_prometheus -> observability.metrics.enable
allow_jaeger -> observability.traces.enable
tracing_flame -> observability.flame.enable
log -> observability.logs.filter
log_colors -> observability.logs.colors
log_format -> observability.logs.format

New config values in these sections will follow.

src/main.rs

@@ -442,7 +442,7 @@ fn routes(config: &Config) -> Router {
                 .put(c2s::send_state_event_for_empty_key_route),
         );
 
-    let router = if config.allow_prometheus {
+    let router = if config.observability.metrics.enable {
         router.route(
             "/metrics",
             get(|| async { observability::METRICS.export() }),