Stringify grouping key values in push client

About a day after fixing this for the job name I realised that I'd made
the same mistake a whole ten lines of code further down the method.

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

Author: Sinjo

lib/prometheus/client/push.rb

@@ -99,6 +99,8 @@ def build_path(job, grouping_key)
         end
 
         grouping_key.each do |label, value|
+          value = value.to_s
+
           if value.include?('/')
             encoded_value = Base64.urlsafe_encode64(value)
             path += "/#{label}@base64/#{encoded_value}"

spec/prometheus/client/push_spec.rb

@@ -101,6 +101,15 @@
       expect(push.path).to eql('/metrics/job/foo')
     end
 
+    it 'converts non-string grouping labels to strings' do
+      push = Prometheus::Client::Push.new(
+        job: 'test-job',
+        grouping_key: { foo: :bar, baz: :qux},
+      )
+
+      expect(push.path).to eql('/metrics/job/test-job/foo/bar/baz/qux')
+    end
+
     it 'encodes the job name in url-safe base64 if it contains `/`' do
       push = Prometheus::Client::Push.new(
         job: 'foo/test-job',