storage: enable the merge queue by default

Start smoking out bugs in range merges by enabling them by default.
This commit will not be backported to v2.1 until we've gained enough
confidence in their stability on master.

Release note: None

docs/generated/settings/settings.html

@@ -31,7 +31,7 @@
 <tr><td><code>kv.raft_log.synchronize</code></td><td>boolean</td><td><code>true</code></td><td>set to true to synchronize on Raft log writes to persistent storage ('false' risks data loss)</td></tr>
 <tr><td><code>kv.range.backpressure_range_size_multiplier</code></td><td>float</td><td><code>2</code></td><td>multiple of range_max_bytes that a range is allowed to grow to without splitting before writes to that range are blocked, or 0 to disable</td></tr>
 <tr><td><code>kv.range_descriptor_cache.size</code></td><td>integer</td><td><code>1000000</code></td><td>maximum number of entries in the range descriptor and leaseholder caches</td></tr>
-<tr><td><code>kv.range_merge.queue_enabled</code></td><td>boolean</td><td><code>false</code></td><td>whether the automatic merge queue is enabled</td></tr>
+<tr><td><code>kv.range_merge.queue_enabled</code></td><td>boolean</td><td><code>true</code></td><td>whether the automatic merge queue is enabled</td></tr>
 <tr><td><code>kv.rangefeed.enabled</code></td><td>boolean</td><td><code>false</code></td><td>if set, rangefeed registration is enabled</td></tr>
 <tr><td><code>kv.snapshot_rebalance.max_rate</code></td><td>byte size</td><td><code>2.0 MiB</code></td><td>the rate limit (bytes/sec) to use for rebalance snapshots</td></tr>
 <tr><td><code>kv.snapshot_recovery.max_rate</code></td><td>byte size</td><td><code>8.0 MiB</code></td><td>the rate limit (bytes/sec) to use for recovery snapshots</td></tr>

pkg/storage/merge_queue.go

@@ -49,7 +49,7 @@ const (
 var MergeQueueEnabled = settings.RegisterBoolSetting(
 	"kv.range_merge.queue_enabled",
 	"whether the automatic merge queue is enabled",
-	false,
+	true,
 )
 
 // MergeQueueInterval is a setting that controls how often the merge queue waits