Measure cross-AZ traffic in safekeepers (#3806)

Create `safekeeper_pg_io_bytes_total` metric to track total amount of
bytes written/read in a postgres connections to safekeepers. This metric
has the following labels:
- `client_az` – availability zone of the connection initiator, or
`"unknown"`
- `sk_az` – availability zone of the safekeeper, or `"unknown"`
- `app_name` – `application_name` of the postgres client
- `dir` – data direction, either `"read"` or `"write"`
- `same_az` – `"true"`, `"false"` or `"unknown"`. Can be derived from
`client_az` and `sk_az`, exists purely for convenience.

This is implemented by passing availability zone in the connection
string, like this: `-c tenant_id=AAA timeline_id=BBB
availability-zone=AZ-1`.

Update ansible deployment scripts to add availability_zone argument
to safekeeper and pageserver in systemd service files.

control_plane/src/safekeeper.rs

@@ -115,6 +115,10 @@ impl SafekeeperNode {
         let datadir = self.datadir_path();
 
         let id_string = id.to_string();
+        // TODO: add availability_zone to the config.
+        // Right now we just specify any value here and use it to check metrics in tests.
+        let availability_zone = format!("sk-{}", id_string);
+
         let mut args = vec![
             "-D",
             datadir.to_str().with_context(|| {
@@ -126,6 +130,8 @@ impl SafekeeperNode {
             &listen_pg,
             "--listen-http",
             &listen_http,
+            "--availability-zone",
+            &availability_zone,
         ];
         if !self.conf.sync {
             args.push("--no-sync");