Validate label names

Up to now, we've only been performing limited validation of label names.
This commit validates that the characters use match the regex
`/\A[a-zA-Z_][a-zA-Z0-9_]*\Z/` as specified by:

https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

lib/prometheus/client/label_set_validator.rb

@@ -7,6 +7,7 @@ module Client
     class LabelSetValidator
       # TODO: we might allow setting :instance in the future
       BASE_RESERVED_LABELS = [:job, :instance, :pid].freeze
+      LABEL_NAME_REGEX = /\A[a-zA-Z_][a-zA-Z0-9_]*\Z/
 
       class LabelSetError < StandardError; end
       class InvalidLabelSetError < LabelSetError; end
@@ -59,9 +60,16 @@ def validate_symbol(key)
       end
 
       def validate_name(key)
-        return true unless key.to_s.start_with?('__')
+        if key.to_s.start_with?('__')
+          raise ReservedLabelError, "label #{key} must not start with __"
+        end
+
+        unless key.to_s =~ LABEL_NAME_REGEX
+          msg = "label name must match /#{LABEL_NAME_REGEX}/"
+          raise InvalidLabelError, msg
+        end
 
-        raise ReservedLabelError, "label #{key} must not start with __"
+        true
       end
 
       def validate_reserved_key(key)

spec/prometheus/client/label_set_validator_spec.rb

@@ -36,6 +36,12 @@
       end.to raise_exception(described_class::ReservedLabelError)
     end
 
+    it 'raises InvalidLabelError if a label key contains invalid characters' do
+      expect do
+        validator.validate_symbols!(:@foo => 'key')
+      end.to raise_exception(described_class::InvalidLabelError)
+    end
+
     it 'raises ReservedLabelError if a label key is reserved' do
       [:job, :instance, :pid].each do |label|
         expect do