feat(promql): do not use regex for string labels (#7669)

In Prometheus, label values are treated as strings when used with the
`=` and `!=` operators, and as regular expressions when used with the
`=~` and `!~` operators.

Injecting and then highlighting all label values as regex leads to a
situation where entirely valid **string** label values containing regex
special characters are mistakenly parsed and highlighted as regex. This
results in syntax errors, causing labels to be highlighted incorrectly.

For example, in `foo{bar=~"[a-z]{1,3}"}`, `{` and `}` are regex special
characters, so regex highlighting is expected. However, in
`foo{path="/foo/{id}"}`, `{` and `}` are just part of the string and
have no special meaning, so the whole value should be highlighted as a
string.

queries/promql/highlights.scm

@@ -36,7 +36,19 @@
 
 (label_name) @variable.member
 
-(label_value) @string.regexp
+((label_name)
+  [
+    "=~"
+    "!~"
+  ]
+  (label_value) @string.regexp)
+
+((label_name)
+  [
+    "="
+    "!="
+  ]
+  (label_value) @string)
 
 (function_name) @function.call
 

queries/promql/injections.scm

@@ -1,6 +1,11 @@
 ((comment) @injection.content
   (#set! injection.language "comment"))
 
-((label_value) @injection.content
+((label_name)
+  [
+    "=~"
+    "!~"
+  ]
+  (label_value) @injection.content
   (#set! injection.language "regex")
   (#offset! @injection.content 0 1 0 -1))