Handle / in job name in Prometheus::Client::Push

This a subtlety I missed when overhauling the Pushgateway client.

While the job name can't be empty like other grouping key labels can, it
can contain `/`, which means we need to base64 encode the value in that
case.

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

lib/prometheus/client/push.rb

@@ -23,7 +23,7 @@ class HttpClientError < HttpError; end
       class HttpServerError < HttpError; end
 
       DEFAULT_GATEWAY = 'http://localhost:9091'.freeze
-      PATH            = '/metrics/job/%s'.freeze
+      PATH = '/metrics'.freeze
       SUPPORTED_SCHEMES = %w(http https).freeze
 
       attr_reader :job, :gateway, :path
@@ -87,7 +87,14 @@ def parse(url)
       end
 
       def build_path(job, grouping_key)
-        path = format(PATH, ERB::Util::url_encode(job))
+        # Job can't be empty, but it can contain `/`, so we need to base64
+        # encode it in that case
+        if job.include?('/')
+          encoded_job = Base64.urlsafe_encode64(job)
+          path = "#{PATH}/job@base64/#{encoded_job}"
+        else
+          path = "#{PATH}/job/#{ERB::Util::url_encode(job)}"
+        end
 
         grouping_key.each do |label, value|
           if value.include?('/')

spec/prometheus/client/push_spec.rb

@@ -93,6 +93,14 @@
       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',
+      )
+
+      expect(push.path).to eql('/metrics/job@base64/Zm9vL3Rlc3Qtam9i')
+    end
+
     it 'encodes grouping key label values containing `/` in url-safe base64' do
       push = Prometheus::Client::Push.new(
         job: 'test-job',