fix doc: metadata.label keys should not allow spaces

- the validation logic in app/messages/metadata_validator_helper.rb
does not allow spaces in keys
- added a relevant test case
- fixed the examples provided in doc

Author: peterhaochen47

docs/v3/source/includes/concepts/_metadata.md.erb

@@ -37,8 +37,8 @@ Only annotations with a prefix (e.g. `company.com/contacts`) are sent to service
 
 Examples may include (but are not limited to):
 
-- `"contact info": "bob@example.com jane@example.com"`
-- `"library versions": "Spring: 5.1, Redis Client: a184098. yaml parser: 38"`
+- `"contact-info": "bob@example.com jane@example.com"`
+- `"library-versions": "Spring: 5.1, Redis Client: a184098. yaml parser: 38"`
 - `"git-sha": "d56fe0367554ae5e878e37ed6c5b9a82f5995512"`
 
 #### Annotation keys

spec/unit/messages/validators/metadata_validator_spec.rb

@@ -81,6 +81,15 @@ def annotations
       end
 
       describe 'invalid keys' do
+        context 'when the key contains a space' do
+          let(:labels) { { 'potato mashed' => 'value' } }
+
+          it 'is invalid' do
+            expect(subject).not_to be_valid
+            expect(subject.errors_on(:metadata)).to include("label key error: 'potato mashed' contains invalid characters")
+          end
+        end
+
         context 'when the key contains one invalid character' do
           # for the 32nd-126th characters, excluding the ones inside of the %r()
           (32.chr..126.chr).to_a.reject { |c| %r{[\w\-._/\s]}.match(c) }.each do |c|