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.

tests/query/injections/promql/regex.promql

@@ -0,0 +1,10 @@
+foo{path=~"^foo$"}[5m] or
+#          ^ @regex
+foo{path!~"[a-zA-Z0-9]{1,3}"}[5m] or
+#          ^ @regex
+foo{path="/api/users/{userId}"}[5m] or
+#         ^ @!regex
+foo{path!="/api/users/{userId}"}[5m]
+#          ^ @!regex
+
+# vim: ft=promql

tests/query/injections/yaml/promql-on-prometheus-rules.yaml

@@ -17,3 +17,17 @@ groups:
     for: 3m
     labels:
       severity: warning
+  - alert: Regex and String matching
+    expr: |
+      foo{path=~"^foo$"}[5m] or foo{path!~"[a-zA-Z0-9]{1,3}"}[5m] or foo{path="/api/users/{userId}"}[5m] or foo{path!="/api/users/{userId}"}[5m]
+    # ^ @promql
+    #            ^ @regex
+    #                                      ^ @regex
+    #                                                                          ^ @!regex
+    #                                                                                                                  ^ @!regex
+    for: 3m
+    labels:
+      severity: warning
+    annotations:
+      title: Foo
+      description: Bar