Fix: rrdtool DS names must be <= 19 chars

sanitizeName now keeps the first 15 chars plus a 3-hex hash suffix
for names longer than 19 chars (rrdtool's hard limit on DS name
length). The previous replace-dots-only version produced names up to
31 chars, which rrdtool rejected with 'invalid DS format'.

test/MetricsSpec.hs

@@ -41,22 +41,41 @@ spec = do
       dsTypeOf (M.Label "hello") `shouldBe` Nothing
 
   describe "sanitizeName" $ do
-    it "replaces dots with underscores" $
-      sanitizeName ("rts.gc.bytes_allocated" :: Text) `shouldBe` "rts_gc_bytes_allocated"
+    it "replaces dots with underscores for short names" $
+      sanitizeName ("rts.gc.cpu_ms" :: Text) `shouldBe` "rts_gc_cpu_ms"
+
+    it "shortens names longer than 19 chars to 15 chars + _ + 3 hex" $ do
+      let result = sanitizeName ("rts.gc.par_balanced_bytes_copied" :: Text)
+      length result `shouldBe` 19
+      take 15 result `shouldBe` "rts_gc_par_bala"
+      drop 15 result `shouldBe` "_" ++ drop 16 result
+
+    it "is deterministic (same input -> same output)" $
+      sanitizeName ("rts.gc.peak_megabytes_allocated" :: Text)
+        `shouldBe` sanitizeName ("rts.gc.peak_megabytes_allocated" :: Text)
 
   describe "buildSchema" $ do
     it "builds sorted DsSpecs from counters and gauges, skipping labels" $
       let sample :: HashMap Text M.Value
           sample = HM.fromList
-            [ ("rts.gc.bytes_allocated", M.Counter 1000)
-            , ("rts.gc.max_bytes_used", M.Gauge 500)
-            , ("rts.gc.label_thing", M.Label "irrelevant")
+            [ ("x.allocated", M.Counter 1000)
+            , ("a.bytes_used", M.Gauge 500)
+            , ("c.label_thing", M.Label "irrelevant")
             ]
       in buildSchema sample `shouldBe`
-           [ DsSpec "rts.gc.bytes_allocated" "rts_gc_bytes_allocated" Derive
-           , DsSpec "rts.gc.max_bytes_used" "rts_gc_max_bytes_used" Gauge
+           [ DsSpec "a.bytes_used" "a_bytes_used" Gauge
+           , DsSpec "x.allocated" "x_allocated" Derive
            ]
 
+    it "produces dsName <= 19 chars for long ekg GC metric names" $
+      let sample :: HashMap Text M.Value
+          sample = HM.fromList
+            [ ("rts.gc.par_balanced_bytes_copied", M.Gauge 1)
+            , ("rts.gc.peak_megabytes_allocated", M.Gauge 2)
+            , ("rts.gc.cumulative_bytes_used", M.Counter 3)
+            ]
+      in map (length . dsName) (buildSchema sample) `shouldSatisfy` all (<= 19)
+
   describe "buildCreateArgs" $ do
     it "builds create argv with mixed DERIVE and GAUGE DSes and RRAs" $
       buildCreateArgs "test.rrd" 10