Added engine stats to FUNCTION STATS command. (#10179)

Added the following statistics (per engine) to FUNCTION STATS command:
* number of functions
* number of libraries

Output example:
```
> FUNCTION stats
1) "running_script"
2) (nil)
3) "engines"
4) 1) "LUA"
   2) 1) "libraries_count"
      2) (integer) 1
      3) "functions_count"
      4) (integer) 1
```

To collect the stats, added a new dictionary to libraries_ctx that contains
for each engine, the engine statistics representing the current libraries_ctx.
Update the stats on:
1. Link library to libraries_ctx
2. Unlink library from libraries_ctx
3. Flushing libraries_ctx

1 files changed, 31 insertions, 1 deletions

diff --git a/tests/unit/functions.tcl b/tests/unit/functions.tcl
index 118362d25..187f7cfc4 100644
--- a/tests/unit/functions.tcl
+++ b/tests/unit/functions.tcl
@@ -245,7 +245,7 @@ start_server {tags {"scripting"}} {
         after 200
         catch {r ping} e
         assert_match {BUSY*} $e
-        assert_match {running_script {name test command {fcall test 0} duration_ms *} engines LUA} [r FUNCTION STATS]
+        assert_match {running_script {name test command {fcall test 0} duration_ms *} engines {*}} [r FUNCTION STATS]
         r function kill
         after 200 ; # Give some time to Lua to call the hook again...
         assert_equal [r ping] "PONG"
@@ -1102,4 +1102,34 @@ start_server {tags {"scripting"}} {
         catch {[r fcall f1 0]} e
         assert_equal  [r fcall get_version_v1 0] [r fcall get_version_v2 0]
     }
+
+    test {FUNCTION - function stats} {
+        r FUNCTION FLUSH
+
+        r FUNCTION load lua test1 {
+            redis.register_function('f1', function() return 1 end)
+            redis.register_function('f2', function() return 1 end)
+        }
+
+        r FUNCTION load lua test2 {
+            redis.register_function('f3', function() return 1 end)
+        }
+
+        r function stats
+    } {running_script {} engines {LUA {libraries_count 2 functions_count 3}}}
+
+    test {FUNCTION - function stats reloaded correctly from rdb} {
+        r debug reload
+        r function stats
+    } {running_script {} engines {LUA {libraries_count 2 functions_count 3}}} {needs:debug}
+
+    test {FUNCTION - function stats delete library} {
+        r function delete test1
+        r function stats
+    } {running_script {} engines {LUA {libraries_count 1 functions_count 1}}}
+
+    test {FUNCTION - function stats cleaned after flush} {
+        r function flush
+        r function stats
+    } {running_script {} engines {LUA {libraries_count 0 functions_count 0}}}
 }