Agent: Add a boolean to the index to indicate agent mode. (#9649)

I would like to avoid extra API call's to determine if we are running in
Agent Mode, so I think we could use this approach.

This is a bootstrap of #9612

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

web/ui/react-app/public/index.html

@@ -10,12 +10,15 @@
     <meta name="theme-color" content="#000000" />
 
     <!--
-      The GLOBAL_CONSOLES_LINK placeholder magic value is replaced during serving by Prometheus
-      and set to the consoles link if it exists. It will render a "Consoles" link in the navbar when
-      it is non-empty.
+      Placeholders replaced by Prometheus during serving:
+      - GLOBAL_CONSOLES_LINK is replaced and set to the consoles link if it exists.
+        It will render a "Consoles" link in the navbar when it is non-empty.
+      - PROMETHEUS_AGENT_MODE is replaced by a boolean indicating if Prometheus is running in agent mode.
+        It true, it will disable querying capacities in the UI and generally adapt the UI to the agent mode.
     -->
     <script>
       const GLOBAL_CONSOLES_LINK='CONSOLES_LINK_PLACEHOLDER';
+      const GLOBAL_PROMETHEUS_AGENT_MODE=PROMETHEUS_AGENT_MODE_PLACEHOLDER;
     </script>
 
     <!--

web/web.go

@@ -32,6 +32,7 @@ import (
 	"regexp"
 	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	template_text "text/template"
@@ -408,6 +409,7 @@ func New(logger log.Logger, o *Options) *Handler {
 		}
 		replacedIdx := bytes.ReplaceAll(idx, []byte("CONSOLES_LINK_PLACEHOLDER"), []byte(h.consolesPath()))
 		replacedIdx = bytes.ReplaceAll(replacedIdx, []byte("TITLE_PLACEHOLDER"), []byte(h.options.PageTitle))
+		replacedIdx = bytes.ReplaceAll(replacedIdx, []byte("PROMETHEUS_AGENT_MODE_PLACEHOLDER"), []byte(strconv.FormatBool(h.options.IsAgent)))
 		w.Write(replacedIdx)
 	}