Indexer metrics ep2 for splunkenterprisereceiver (#30757)

**Description:** 
Adds additional indexer metrics to Splunk Enterprise receiver obtained
from ad-hoc searches against Indexer or Cluster Manager instances.

Disables by default API endpoint searches that only return results for
the specific instance which is having its API called. These can be
re-enabled in config if those metrics are specifically required.

Generated tests and docs for these additional metrics

**Link to tracking Issue:**
[30704](#30704)

**Testing:** Tests generated with mdatagen

**Documentation:** Documentation generated with mdatagen

.chloggen/indexer-metrics-2-splunkent.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: breaking
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: splunkenterprisereceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "adds additional metrics specific to indexers"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [30704]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]