Add a merge unit. (#110)

This PR adds a new merge unit that merges the payload from all its healthy
sources.

doc/manual/source/manual-page.rst

@@ -239,6 +239,16 @@ random
       A boolean value specifying whether the unit should pick a source unit
       at random. If the value is ``false`` or not given, the source units are
       picked in the order given.
+
+Merge Unit
+----------
+
+A unit of type ``"merge"`` will merge the data from all data sets of its
+source units. It has the following configuration options:
+
+sources
+      A list of strings each containing the name of a unit to use as a
+      source.
 
 
 SLURM Unit

etc/rtrtr.conf

@@ -94,6 +94,12 @@ sources = [ "local-3323", "local-3324", "cloudflare-json" ]
 random = false
 
 
+# We can also merge the data from multiple sources
+[units.merged-rtr]
+type = "merge"
+sources = [ "local-3323", "local-3324", "cloudflare-json" ]
+
+
 # Local exceptions can be applied, too.
 [units.slurm]
 type = "slurm"
@@ -115,6 +121,14 @@ listen = [ "127.0.0.1:9001" ]
 unit = "any-rtr"
 
 
+# Let’s have a target for the merged data, too.
+[targets.local-9002]
+type = "rtr"
+listen = [ "127.0.0.1:9002" ]
+unit = "merged-rtr"
+
+
+# We can also provide the data as JSON over HTTP.
 [targets.http-json]
 type = "http"
 path = "/json"