add more prometheus metrics; breaking changes:

* cpp_uptime is now a gauge
* cpp_bans is now cpp_active_bans (and also a gauge)

and other related fixes:
* stop emitting invalid cpp_disk_size/free for offline volumes
* support overriding the spec-mandatory mimetype with ?mime=foo

copyparty/httpsrv.py

@@ -128,6 +128,9 @@ class HttpSrv(object):
 
         self.u2fh = FHC()
         self.metrics = Metrics(self)
+        self.nreq = 0
+        self.nsus = 0
+        self.nban = 0
         self.srvs: list[socket.socket] = []
         self.ncli = 0  # exact
         self.clients: set[HttpConn] = set()  # laggy