prometheus_text_format: Avoid allocation in has_special_char/1

`prometheus_text_format:has_special_char/1` is called very often when
a registry contains many metrics with label pairs. We can use
`binary:match/2` to search within a label binary for the special
characters (newline, backslash and double-quote) without allocation.

The old code using binary match syntax creates a match context every
time the function is called (except, not recursion - then the match
context is reused). A match context allocates 5 words to the process
heap when it is created. When matching many many binaries this scales to
create a noticeable amount of short-lived garbage.

In comparison `binary:match/2` with a precompiled match pattern does not
allocate. The BIF for it is also very well optimized, using `memchr`
since OTP 22.

Author: the-mikedavis

src/formats/prometheus_text_format.erl

@@ -289,12 +289,15 @@ escape_label_char(X) ->
 
 ?DOC(false).
 -spec has_special_char(binary()) -> boolean().
-has_special_char(<<C:8, _/bitstring>>) when C =:= $\\; C =:= $\n; C =:= $" ->
-    true;
-has_special_char(<<_:8, Rest/bitstring>>) ->
-    has_special_char(Rest);
-has_special_char(<<>>) ->
-    false.
+has_special_char(Subject) when is_binary(Subject) ->
+    case get(prometheus_text_format_escape_pattern) of
+        undefined ->
+            Pat = binary:compile_pattern([<<$\\>>, <<$\n>>, <<$">>]),
+            put(prometheus_text_format_escape_pattern, Pat),
+            has_special_char(Subject);
+        Pat ->
+            binary:match(Subject, Pat) /= nomatch
+    end.
 
 ?DOC(false).
 escape_string(Fun, Str) when is_binary(Str) ->