start integrating a KV store for metrics

the local metrics storage was not very satisfying: metrics are pre
aggregated, so requesting them gives the last instant value, they are
deleted regularly to preserve memory so not much data is kept, and
requesting them through the `metrics` command line option gathers all of
the data in one object (which can get large with many applications)
then deletes everything.

This commit introduces the sled key value database to store current and
previous metrics, and to query them in a more flexible way

command/src/proxy.rs

@@ -60,6 +60,15 @@ pub struct AppMetricsData {
   pub backends: BTreeMap<String, BTreeMap<String, FilteredData>>,
 }
 
+impl AppMetricsData {
+    pub fn new() -> Self {
+        AppMetricsData {
+            data: BTreeMap::new(),
+            backends: BTreeMap::new(),
+        }
+    }
+}
+
 #[derive(Debug,Clone,PartialEq,Eq,Hash,Serialize,Deserialize)]
 #[serde(tag = "type", content = "data", rename_all = "SCREAMING_SNAKE_CASE")]
 pub enum FilteredData {

lib/Cargo.toml

@@ -47,6 +47,8 @@ hashbrown = "0.9"
 regex = "1.1"
 cookie-factory = "^0.3"
 hpack = "^0.3"
+sled = "0.34"
+tempfile = "^3"
 
 [dev-dependencies]
 tiny_http = "^0.8"