YSQL: Rename SingleShardTransaction metric name

Summary:
Today, the platform team relabels the name of metrics published.
The relabel [[ https://github.com/yugabyte/yugabyte-db/blob/master/managed/devops/replicated.yml#L266 | rule ]] can't parse the "_"s in existing names correctly
and it caused that the Prometheus server didn't group ysqlserver metrics properly.

Fix this by removing the unnecessary "_"s from the metric names.

Test Plan:
Jenkins: build-only

Run `curl 127.0.0.1:13000/prometheus-metrics`

```
handler_latency_yb_ysqlserver_SQLProcessor_SingleShardTransactions_count{metric_id="yb.ysqlserver",metric_type="server",exported_instance="Steves-MacBook-Pro.local:9000"} 0 1644528212230
handler_latency_yb_ysqlserver_SQLProcessor_SingleShardTransactions_sum{metric_id="yb.ysqlserver",metric_type="server",exported_instance="Steves-MacBook-Pro.local:9000"} 0 1644528212230
```

Started a local prometheus server and verified with query
`rpc_latency_count{server_type="yb_ysqlserver"}`

Reviewers: amalyshev, fizaa

Reviewed By: fizaa

Subscribers: fizaa, smishra, yql

Differential Revision: https://phabricator.dev.yugabyte.com/D15392

src/postgres/contrib/yb_pg_metrics/yb_pg_metrics.c

@@ -176,8 +176,7 @@ set_metric_names(void)
   strcpy(ybpgm_table[Commit].name, YSQL_METRIC_PREFIX "CommitStmt");
   strcpy(ybpgm_table[Rollback].name, YSQL_METRIC_PREFIX "RollbackStmt");
   strcpy(ybpgm_table[Other].name, YSQL_METRIC_PREFIX "OtherStmts");
-  strcpy(ybpgm_table[Single_Shard_Transaction].name,
-         YSQL_METRIC_PREFIX "Single_Shard_Transactions");
+  strcpy(ybpgm_table[Single_Shard_Transaction].name, YSQL_METRIC_PREFIX "SingleShardTransactions");
   strcpy(ybpgm_table[Transaction].name, YSQL_METRIC_PREFIX "Transactions");
   strcpy(ybpgm_table[AggregatePushdown].name, YSQL_METRIC_PREFIX "AggregatePushdowns");
   strcpy(ybpgm_table[CatCacheMisses].name, YSQL_METRIC_PREFIX "CatalogCacheMisses");