lvm: add support for "all" checks

It seems rather inconvenient to have to specify all those switches by
hand. Right now, I need to specify -gpst to get all the stats and, if
some new metric get added (e.g. prometheus-community#159 hopefully), I need to remember to
update my callers to match. This is annoying.

Let's make a -a that's going to do it all for us. Hell, i'd make it
the default, personally, if I wasn't worried about breaking people's
stuff...

This obviously conflicts with other PRs that might add another flag
like this, like prometheus-community#159. Please prioritize the latter.

Signed-off-by: Antoine Beaupré <anarcat@debian.org>

lvm-prom-collector

@@ -48,8 +48,14 @@ snapshots=false
 physical=false
 groups=false
 
-while getopts "htpsg" opt; do
+while getopts "ahtpsg" opt; do
   case $opt in
+    a)
+      thin_pools=true
+      snapshots=true
+      physical=true
+      groups=true
+      ;;
     p)
       physical=true
       ;;