logstash: Export config.lib.logstash.mk{Float,Hash,NameValuePairs}.

This allows hiding the implementation details for how to represent logstash
config types that don't directly map to nix expressions, particularly floats,
hashes, and name-value pair sets with repeated names. Instead of setting
__type and value directly, the user now uses these convenience functions to
generate their logstash config.

modules/services/logging/logstash.nix

@@ -86,18 +86,17 @@ in
       inputConfig = mkOption {
         default = {};
         description = ''
-          An attr set representing a logstash configuration's input section.
+          A nixexpr representing a logstash configuration's input section.
           logstash configs are name-value pairs, where values can be bools,
           strings, numbers, arrays, hashes, or other name-value pairs,
           and names are strings that can be repeated. name-value pairs with no
-          repeats are represented by attr sets. name-value pairs with repeats
-          are represented by an attrset with attr "__type" = "repeated" and
-          attr "values" as a list of {name; value;} attrsets.
-          bools, strings, ints, and arrays are mapped directly. Floats are
-          represented as an attrset with attr "__type" = "float" and attr value
-          set to the string representation of the float. Hashes are represented
-          with attr "__type" = "hash" and attr value set to an attr set
-          corresponding to the hash.
+          repeats are represented by attr sets. bools, strings, ints, and
+          arrays are mapped directly. name-value pairs with repeats can be
+          generated by the config.lib.logstash.mkNameValuePairs function, which
+          takes a list of attrsets and combines them while preserving attribute
+          name duplicates if they occur. Similarly, there are the mkFloat and
+          mkHash functions, which take a string representation of a float and an
+          attrset, respectively.
         '';
         merge = mergeConfigs;
       };
@@ -125,7 +124,15 @@ in
 
   ###### implementation
 
-  config = mkIf cfg.enable {
+  config = mkMerge [ {
+    lib.logstash = {
+      mkFloat = stringRep: { __type = "float"; value = stringRep; };
+
+      mkHash = attrs: { __type = "hash"; value = attrs; };
+
+      mkNameValuePairs = mergeConfigs;
+    };
+  } ( mkIf cfg.enable {
     # Always log to stdout
     services.logstash.outputConfig = { stdout = {}; };
 
@@ -148,5 +155,5 @@ in
         }
       ''}";
     };
-  };
+  })];
 }