[Uptime] Kibana always tries to save service monitors to all locations, not only the selected ones

[lucasfcosta]

Kibana version: Top of main (SHA 05b730858885a89e84207a55a9b602343a482285)

Elasticsearch version: 8.13.0-SNAPSHOT (Doesn't impact this issue)

Browser version: 101.0.4951.54 (Official Build) (x86_64)

Original install method (e.g. download page, yum, from source, etc.): Running Kibana from source and ES using elastic-package stack.

Describe the bug: Kibana always tries to save service monitors to all locations, not only the selected ones.

Besides increasing the usage of all locations, it can also cause errors if there's a duplicated back-end URL in the manifest, even if the duplicated locations are not selected.

Steps to reproduce:

1. Start three HTTP echo servers using the command below:

   docker run -p 8033:80 -t mendhak/http-https-echo
   docker run -p 8034:80 -t mendhak/http-https-echo
   docker run -p 8035:80 -t mendhak/http-https-echo
   

2. Create a new manifest.json with three locations, one for each HTTP echo server

       {
     "throttling": {
       "download": 20,
       "upload": 10
     },
     "locations": {
       "Bin One": {
         "url": "http://localhost:8033",
         "geo": {
           "name": "BIN ONE",
           "location": {"lat": 41.25, "lon": -95.86}
         },
         "status": "experimental"
       },
       "Bin Two": {
         "url": "http://localhost:8034",
         "geo": {
           "name": "BIN TWO",
           "location": {"lat": 41.25, "lon": -95.86}
         },
         "status": "experimental"
       },
       "Bin Three": {
         "url": "http://localhost:8035",
         "geo": {
           "name": "BIN ONE",
           "location": {"lat": 41.25, "lon": -95.86}
         },
         "status": "experimental"
       }
     }
   }

3. Save that manifest to a service_manifest folder as manifest.json and serve it using the command below

   npx http-server service_manifest --port 8081
   

4. Set Kibana's manifestUrl so that it fetches locations from your local manifest

   # Make sure to comment the xpack.uptime.service.devUrl setting
   xpack.uptime.service.manifestUrl: http://localhost:8081/manifest.json
   

5. Start Kibana and try to create a monitor, but select only the first location. You'll see that all three locations receive requests.

Expected behavior: Monitors should only be created in the selected locations.

Any additional context: I found this problem while investigating why one gets configmap already exists when saving a monitor if there are two locations with the same URL, even if both locations aren't selected.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[emilioalvap]

PostFF testing LGTM.

Tested following the instructions provided in the PR, Kibana is now pinging selected locations only.