Disable default search metrics (open-telemetry#39068)

#### Description
Removing default metrics (i.e. setting all metrics to `enabled: false`).
This is primarily to prevent unwanted ad-hoc searches from running on
Splunk Enterprise deployments which could have unforeseen side effects.

#### Documentation
The `README.md` was updated to reflect these changes.

Author: shalper2

.chloggen/disable-default-search-metrics.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: "disabled default metrics except for splunkHealth to ensure scrapes run on Splunk instance are opt-in"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [39068]
+
+# (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]

receiver/splunkenterprisereceiver/README.md

@@ -20,6 +20,8 @@ jobs.
 
 ## Configuration
 
+**By default the Splunk Enterprise receiver is not configured to gather any metrics other than `splunk.health`**
+
 The following settings are required, omitting them will either cause your receiver to fail to compile or result in 4/5xx return codes during scraping. 
 
 **NOTE:** These must be set for each Splunk instance type (indexer, search head, or cluster master) from which you wish to pull metrics. At present, only one of each type is accepted, per configured receiver instance. This means, for example, that if you have three different "indexer" type instances that you would like to pull metrics from you will need to configure three different `splunkenterprise` receivers for each indexer node you wish to monitor.