document label_join (#825)

content/docs/querying/functions.md

@@ -241,6 +241,17 @@ or a function aggregating over time (any function ending in `_over_time`),
 always take a `irate()` first, then aggregate. Otherwise `irate()` cannot detect
 counter resets when your target restarts.
 
+## `label_join()`
+
+For each timeseries in `v`, `label_join(v instant-vector, dst_label string, separator string, src_label_1 string, src_label_2 string, ...)` joins all the values of all the `src_labels` 
+using `separator` and returns the timeseries with the label `dst_label` containing the joined value.
+There can be any number of `src_labels` in this function.
+
+This example will return a vector with each time series having a `foo` label with the value `a,b,c` added to it:
+```
+label_join(up{job="api-server",src1="a",src2="b",src3="c"}, "foo", ",", "src1", "src2", "src3")
+```
+
 ## `label_replace()`
 
 For each timeseries in `v`, `label_replace(v instant-vector, dst_label string, replacement string,