Add --id flag to amtool silence query (prometheus#3241)

In the Web UI, we have a UI to get information on a given silence
through its ID. This functionality is missing from amtool however,
leading to the necessity to pull _all_ the silenced, and filter with
`grep` or similar.

This commit adds in a `--id` flag to the `silence query` command, which
allows specifying an ID to match on, matching the functionality of the Web UI.

Signed-off-by: sinkingpoint <colin@quirl.co.nz>

cli/silence_query.go

@@ -31,6 +31,7 @@ type silenceQueryCmd struct {
 	expired   bool
 	quiet     bool
 	createdBy string
+	ID        string
 	matchers  []string
 	within    time.Duration
 }
@@ -86,6 +87,7 @@ func configureSilenceQueryCmd(cc *kingpin.CmdClause) {
 	queryCmd.Flag("expired", "Show expired silences instead of active").BoolVar(&c.expired)
 	queryCmd.Flag("quiet", "Only show silence ids").Short('q').BoolVar(&c.quiet)
 	queryCmd.Flag("created-by", "Show silences that belong to this creator").StringVar(&c.createdBy)
+	queryCmd.Flag("id", "Get a single silence by its ID").StringVar(&c.ID)
 	queryCmd.Arg("matcher-groups", "Query filter").StringsVar(&c.matchers)
 	queryCmd.Flag("within", "Show silences that will expire or have expired within a duration").DurationVar(&c.within)
 	queryCmd.Action(execWithTimeout(c.query))
@@ -133,6 +135,10 @@ func (c *silenceQueryCmd) query(ctx context.Context, _ *kingpin.ParseContext) er
 		if c.createdBy != "" && *silence.CreatedBy != c.createdBy {
 			continue
 		}
+		// Skip silences if the ID doesn't match.
+		if c.ID != "" && c.ID != *silence.ID {
+			continue
+		}
 
 		displaySilences = append(displaySilences, *silence)
 	}