doc: make sql.index_recommendation.drop_unused_duration public

Make the cluster setting `sql.index_recommendation.drop_unused_duration`
public.

Fixes #104734

Release note (sql change): Make the cluster setting
`sql.index_recommendation.drop_unused_duration` public for docs.

docs/generated/settings/settings-for-tenants.txt

@@ -246,6 +246,7 @@ sql.distsql.temp_storage.workmem	byte size	64 MiB	maximum amount of memory in by
 sql.guardrails.max_row_size_err	byte size	512 MiB	maximum size of row (or column family if multiple column families are in use) that SQL can write to the database, above which an error is returned; use 0 to disable	application
 sql.guardrails.max_row_size_log	byte size	64 MiB	maximum size of row (or column family if multiple column families are in use) that SQL can write to the database, above which an event is logged to SQL_PERF (or SQL_INTERNAL_PERF if the mutating statement was internal); use 0 to disable	application
 sql.hash_sharded_range_pre_split.max	integer	16	max pre-split ranges to have when adding hash sharded index to an existing table	application
+sql.index_recommendation.drop_unused_duration	duration	168h0m0s	the index unused duration at which we begin to recommend dropping the index	application
 sql.insights.anomaly_detection.enabled	boolean	true	enable per-fingerprint latency recording and anomaly detection	application
 sql.insights.anomaly_detection.latency_threshold	duration	50ms	statements must surpass this threshold to trigger anomaly detection and identification	application
 sql.insights.anomaly_detection.memory_limit	byte size	1.0 MiB	the maximum amount of memory allowed for tracking statement latencies	application

docs/generated/settings/settings.html

@@ -195,6 +195,7 @@
 <tr><td><div id="setting-sql-guardrails-max-row-size-err" class="anchored"><code>sql.guardrails.max_row_size_err</code></div></td><td>byte size</td><td><code>512 MiB</code></td><td>maximum size of row (or column family if multiple column families are in use) that SQL can write to the database, above which an error is returned; use 0 to disable</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
 <tr><td><div id="setting-sql-guardrails-max-row-size-log" class="anchored"><code>sql.guardrails.max_row_size_log</code></div></td><td>byte size</td><td><code>64 MiB</code></td><td>maximum size of row (or column family if multiple column families are in use) that SQL can write to the database, above which an event is logged to SQL_PERF (or SQL_INTERNAL_PERF if the mutating statement was internal); use 0 to disable</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
 <tr><td><div id="setting-sql-hash-sharded-range-pre-split-max" class="anchored"><code>sql.hash_sharded_range_pre_split.max</code></div></td><td>integer</td><td><code>16</code></td><td>max pre-split ranges to have when adding hash sharded index to an existing table</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
+<tr><td><div id="setting-sql-index-recommendation-drop-unused-duration" class="anchored"><code>sql.index_recommendation.drop_unused_duration</code></div></td><td>duration</td><td><code>168h0m0s</code></td><td>the index unused duration at which we begin to recommend dropping the index</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
 <tr><td><div id="setting-sql-insights-anomaly-detection-enabled" class="anchored"><code>sql.insights.anomaly_detection.enabled</code></div></td><td>boolean</td><td><code>true</code></td><td>enable per-fingerprint latency recording and anomaly detection</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
 <tr><td><div id="setting-sql-insights-anomaly-detection-latency-threshold" class="anchored"><code>sql.insights.anomaly_detection.latency_threshold</code></div></td><td>duration</td><td><code>50ms</code></td><td>statements must surpass this threshold to trigger anomaly detection and identification</td><td>Serverless/Dedicated/Self-Hosted</td></tr>
 <tr><td><div id="setting-sql-insights-anomaly-detection-memory-limit" class="anchored"><code>sql.insights.anomaly_detection.memory_limit</code></div></td><td>byte size</td><td><code>1.0 MiB</code></td><td>the maximum amount of memory allowed for tracking statement latencies</td><td>Serverless/Dedicated/Self-Hosted</td></tr>

pkg/sql/idxusage/index_usage_stats_rec.go

@@ -45,9 +45,10 @@ const defaultUnusedIndexDuration = 7 * 24 * time.Hour
 var DropUnusedIndexDuration = settings.RegisterDurationSetting(
 	settings.ApplicationLevel,
 	"sql.index_recommendation.drop_unused_duration",
-	"the index unuse duration at which we begin to recommend dropping the index",
+	"the index unused duration at which we begin to recommend dropping the index",
 	defaultUnusedIndexDuration,
 	settings.NonNegativeDuration,
+	settings.WithPublic,
 )
 
 const indexExceedUsageDurationReasonPlaceholder = "This index has not been used in over %sand can be removed for better write performance."