lib/logstorage: add non-empty if (...) condition to automatically generated result names in stats pipe

This allows executing queries with `stats` pipe, which calculate multiple results with the same functions,
but with different `if (...)` conditions. For example:

  _time:5m | count(), count() if (error)

Previously such queries couldn't be executed becasue automatically generated name for the second result
didn't include `if (error)`, so names for both results were identical - `count(*)`.

docs/VictoriaLogs/CHANGELOG.md

@@ -17,6 +17,7 @@ according to [these docs](https://docs.victoriametrics.com/victorialogs/quicksta
 
 * FEATURE: [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/): accept Unix timestamps in seconds in the ingested logs.
 * FEATURE: [`sort` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#sort-pipe): allow using `order` alias instead of `sort`. For example, `_time:5s | order by (_time)` query works the same as `_time:5s | sort by (_time)`.
+* FEATURE: [`stats` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#stats-pipe): allow using multiple identical [stats functions](https://docs.victoriametrics.com/victorialogs/logsql/#stats-pipe-functions) with distinct [filters](https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters) and automatically generated result names. For example, `_time:5m | count(), count() if (error)` query works as expected now, e.g. it returns two results over the last 5 minutes: the total number of logs and the number of logs with `error` [word](https://docs.victoriametrics.com/victorialogs/logsql/#word). Previously this query couldn't be executed because the `if (...)` condition wasn't included in the automatically generate result name, so both results had the same name - `count(*)`.
 
 ## [v0.31.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v0.31.0-victorialogs)
 

lib/logstorage/parser_test.go

@@ -1025,6 +1025,7 @@ func TestParseQuerySuccess(t *testing.T) {
 	f(`* | stats count('') foo`, `* | stats count(_msg) as foo`)
 	f(`* | stats count(foo) ''`, `* | stats count(foo) as _msg`)
 	f(`* | count()`, `* | stats count(*) as "count(*)"`)
+	f(`* | count(), count() if (foo)`, `* | stats count(*) as "count(*)", count(*) if (foo) as "count(*) if (foo)"`)
 
 	// stats pipe count_empty
 	f(`* | stats count_empty() x`, `* | stats count_empty(*) as x`)

lib/logstorage/pipe_stats.go

@@ -577,6 +577,9 @@ func parsePipeStats(lex *lexer, needStatsKeyword bool) (*pipeStats, error) {
 		resultName := ""
 		if lex.isKeyword(",", "|", ")", "") {
 			resultName = sf.String()
+			if f.iff != nil {
+				resultName += " " + f.iff.String()
+			}
 		} else {
 			if lex.isKeyword("as") {
 				lex.nextToken()