log: add pretty printer module

Because the debug module doesn't use best practices, pretty-printers using the
Format module are not ergonomic to use when they are needed for constructing
loglines.

Provide a module for adapters, containing Mtime's spans for the time being.

Signed-off-by: Pau Ruiz Safont <[email protected]>

ocaml/libs/log/debug.ml

@@ -352,3 +352,5 @@ functor
       try f ()
       with e -> log_backtrace_internal ~level:Syslog.Debug ~msg:"debug" e ()
   end
+
+module Pp = struct let mtime_span () = Fmt.str "%a" Mtime.Span.pp end

ocaml/libs/log/debug.mli

@@ -88,3 +88,7 @@ module Make : functor (_ : BRAND) -> DEBUG
 val is_disabled : string -> Syslog.level -> bool
 (** [is_disabled brand level] returns [true] if logging for [brand] at [level]
     is disabled, * otherwise returns [false]. *)
+
+module Pp : sig
+  val mtime_span : unit -> Mtime.Span.t -> string
+end

ocaml/libs/log/dune

@@ -6,6 +6,8 @@
    (names syslog_stubs))
  (libraries
    astring
+   fmt
+   mtime
    logs
    threads.posix
    xapi-backtrace

ocaml/xapi-guard/lib/disk_cache.ml

@@ -398,9 +398,9 @@ end = struct
     let* failed = retry true in
     ( if failed then
         let elapsed = Mtime_clock.count counter in
-        D.debug "%s: Pushed %s after trying for %s" __FUN
+        D.debug "%s: Pushed %s after trying for %a" __FUN
           (print_key (uuid, timestamp, key))
-          (Fmt.to_to_string Mtime.Span.pp elapsed)
+          Debug.Pp.mtime_span elapsed
     ) ;
     Lwt.return_unit