Fix PromQL lexer to support values enclosed within single quotes

Values for labels could now be enclosed within single or double quotes.

Author: pabluk

Diff for: lexers/p/promql.go

@@ -44,7 +44,7 @@ func promqlRules() Rules {
 			{`\n`, TextWhitespace, nil},
 			{`\s+`, TextWhitespace, nil},
 			{`,`, Punctuation, nil},
-			{`([_a-zA-Z][a-zA-Z0-9_]*?)(\s*?)(=~|!=|=|~!)(\s*?)(")(.*?)(")`, ByGroups(NameLabel, TextWhitespace, Operator, TextWhitespace, Punctuation, LiteralString, Punctuation), nil},
+			{`([_a-zA-Z][a-zA-Z0-9_]*?)(\s*?)(=~|!=|=|~!)(\s*?)("|')(.*?)("|')`, ByGroups(NameLabel, TextWhitespace, Operator, TextWhitespace, Punctuation, LiteralString, Punctuation), nil},
 		},
 		"range": {
 			{`\]`, Punctuation, Pop(1)},

Diff for: lexers/testdata/promql.actual

@@ -28,3 +28,6 @@ label_replace(
     "instance",
     ".*"
 )
+
+# Values for labels enclosed within single quotes
+metric_test_app{app='turtle',proc='web'}

Diff for: lexers/testdata/promql.expected

@@ -165,5 +165,20 @@
   {"type":"Punctuation","value":"\""},
   {"type":"TextWhitespace","value":"\n"},
   {"type":"Operator","value":")"},
+  {"type":"TextWhitespace","value":"\n\n"},
+  {"type":"CommentSingle","value":"# Values for labels enclosed within single quotes"},
+  {"type":"TextWhitespace","value":"\n"},
+  {"type":"NameVariable","value":"metric_test_app"},
+  {"type":"Punctuation","value":"{"},
+  {"type":"NameLabel","value":"app"},
+  {"type":"Operator","value":"="},
+  {"type":"Punctuation","value":"'"},
+  {"type":"LiteralString","value":"turtle"},
+  {"type":"Punctuation","value":"',"},
+  {"type":"NameLabel","value":"proc"},
+  {"type":"Operator","value":"="},
+  {"type":"Punctuation","value":"'"},
+  {"type":"LiteralString","value":"web"},
+  {"type":"Punctuation","value":"'}"},
   {"type":"TextWhitespace","value":"\n"}
 ]