Fix URI escaping of spaces in push.rb

When `URI.escape` was deprecated, we switched to using `CGI.escape` to
construct the request path in push.rb.

The problem with that is that it mishandles spaces, encoding them as `+`
rather than `%20`. This leads to literal `+` characters in metric
labels, which is a bug.

This commit uses `ERB::Util.url_encode` instead, which gives us the
behaviour we want.

It's not the part of the standard library I expected to use for this,
but it's the only one that seems to have the behaviour we want and that
hasn't disappeared in Ruby 3.0. `WEBrick::HTTPUtils.escape` seems
similar on the surface, but doesn't encode `&`. Presumably it's intended
as a best-effort way to encode an entire URL, and has to assume that any
`&` character is a separator between query params.

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

lib/prometheus/client/push.rb

@@ -3,7 +3,7 @@
 require 'thread'
 require 'net/http'
 require 'uri'
-require 'cgi'
+require 'erb'
 
 require 'prometheus/client'
 require 'prometheus/client/formats/text'
@@ -72,9 +72,9 @@ def parse(url)
 
       def build_path(job, instance)
         if instance && !instance.empty?
-          format(INSTANCE_PATH, CGI::escape(job), CGI::escape(instance))
+          format(INSTANCE_PATH, ERB::Util::url_encode(job), ERB::Util::url_encode(instance))
         else
-          format(PATH, CGI::escape(job))
+          format(PATH, ERB::Util::url_encode(job))
         end
       end
 

spec/prometheus/client/push_spec.rb

@@ -91,7 +91,7 @@
     it 'escapes non-URL characters' do
       push = Prometheus::Client::Push.new(job: 'bar job', instance: 'foo <my instance>')
 
-      expected = '/metrics/job/bar+job/instance/foo+%3Cmy+instance%3E'
+      expected = '/metrics/job/bar%20job/instance/foo%20%3Cmy%20instance%3E'
       expect(push.path).to eql(expected)
     end
   end