Cherry-pick #19537 to 7.8: Fix empty field error in the iis/application pool metricset

[narph]

Cherry-pick of PR #19537 to 7.8 branch. Original message:

What does this PR do?

Fixes errors as:

"metricset":{"name":"application_pool","period":10000},"service":{"type":"iis"}}, Private:interface {}(nil), TimeSeries:true}, Flags:0x0, Cache:publisher.EventCache{m:common.MapStr(nil)}} (status=400): {"type":"mapper_parsing_exception","reason":"failed to parse","caused_by":{"type":"illegal_argument_exception","reason":"field name cannot be an empty string"}}

also improves refresh functionality

Why is it important?

Errors

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Run the IIS module wit the application pool metricset enabled

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)