feat(logcli): query from absolute timestamp (#736)

* feat(logcli): query from absolute timestamp

Adds a new flag (`--from`) as to complement `--since`.
While since subtracts a relative duration from the current time, from allows to
specify the absolute start of the lookback window.

Note: `--from` takes precedence over `--since`, but only if set.

* fix(logcli): use RFC3339Nano for -from

To comply with Prometheus, this flag now honors Nanoseconds as well (was using
RFC3339 so far)

cmd/logcli/main.go

@@ -24,6 +24,7 @@ var (
 	regexpStr       = queryCmd.Arg("regex", "").String()
 	limit           = queryCmd.Flag("limit", "Limit on number of entries to print.").Default("30").Int()
 	since           = queryCmd.Flag("since", "Lookback window.").Default("1h").Duration()
+	from            = queryCmd.Flag("from", "Start looking for logs at this absolute time").String()
 	forward         = queryCmd.Flag("forward", "Scan forwards through logs.").Default("false").Bool()
 	tail            = queryCmd.Flag("tail", "Tail the logs").Short('t').Default("false").Bool()
 	delayFor        = queryCmd.Flag("delay-for", "Delay in tailing by number of seconds to accumulate logs").Default("0").Int()

cmd/logcli/query.go

@@ -25,6 +25,14 @@ func doQuery() {
 
 	end := time.Now()
 	start := end.Add(-*since)
+	if *from != "" {
+		var err error
+		start, err = time.Parse(time.RFC3339Nano, *from)
+		if err != nil {
+			log.Fatalf("error parsing date '%s': %s", *from, err)
+		}
+	}
+
 	d := logproto.BACKWARD
 	if *forward {
 		d = logproto.FORWARD