fix validation and defaults for elasticsearch-idx retry-interval

fixes crashes like:

```
Aug 23 09:41:39 dieter-mt1 metrictank[20586]: panic: non-positive
interval for NewTicker
Aug 23 09:41:39 dieter-mt1 metrictank[20586]: goroutine 70 [running]:
Aug 23 09:41:39 dieter-mt1 metrictank[20586]: panic(0xa32980,
0xc82000e9f0)
Aug 23 09:41:39 dieter-mt1 metrictank[20586]:
/usr/local/go/src/runtime/panic.go:481 +0x3e6
Aug 23 09:41:39 dieter-mt1 metrictank[20586]: time.NewTicker(0x0, 0x14)
Aug 23 09:41:39 dieter-mt1 metrictank[20586]:
/usr/local/go/src/time/tick.go:23 +0xe4
Aug 23 09:41:39 dieter-mt1 metrictank[20586]:
github.com/raintank/metrictank/idx/elasticsearch.(*RetryBuffer).run(0xc820050230,
0x0)
Aug 23 09:41:39 dieter-mt1 metrictank[20586]:
/home/ubuntu/.go_workspace/src/github.com/raintank/metrictank/idx/elasticsearch/elasticsearch.go:114
+0x5d
Aug 23 09:41:39 dieter-mt1 metrictank[20586]: created by
github.com/raintank/metrictank/idx/elasticsearch.NewRetryBuffer
Aug 23 09:41:39 dieter-mt1 metrictank[20586]:
/home/ubuntu/.go_workspace/src/github.com/raintank/metrictank/idx/elasticsearch/elasticsearch.go:67
+0x176
```

docs/metadata.md

@@ -21,8 +21,8 @@ enabled = false
 index = metric
 # Elasticsearch host addresses (multiple hosts can be specified as comma-separated list)
 hosts = localhost:9200
-# how often the retry buffer should be flushed to ES
-retry-interval = 1h
+# how often the retry buffer should be flushed to ES. Valid units are 's', 'm', 'h'.
+retry-interval = 10m
 # max number of concurrent connections to ES
 max-conns = 20
 # max number of docs to keep in the BulkIndexer buffer

idx/elasticsearch/elasticsearch.go

@@ -3,6 +3,7 @@ package elasticsearch
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"strings"
@@ -153,6 +154,10 @@ func New() *EsIdx {
 
 func (e *EsIdx) Init(stats met.Backend) error {
 
+	if esRetryInterval < time.Second {
+		return errors.New("Invalid retry-interval.  Valid units are 's', 'm', 'h'. Must be at least 1 second")
+	}
+
 	log.Info("initializing EsIdx. Hosts=%s", esHosts)
 	if err := e.MemoryIdx.Init(stats); err != nil {
 		return err

metrictank-sample.ini

@@ -197,8 +197,8 @@ hosts = localhost:9200
 # http basic auth
 user =
 pass =
-# how often the retry buffer should be flushed to ES
-retry-interval = 10min
+# how often the retry buffer should be flushed to ES. Valid units are "s", "m", "h"
+retry-interval = 10m
 # max number of concurrent connections to ES
 max-conns = 20
 # max numver of docs to keep in the BulkIndexer buffer

scripts/config/metrictank.ini

@@ -194,8 +194,8 @@ hosts = localhost:9200
 # http basic auth
 user =
 pass =
-# how often the retry buffer should be flushed to ES
-retry-interval = 10min
+# how often the retry buffer should be flushed to ES. Valid units are "s", "m", "h".
+retry-interval = 10m
 # max number of concurrent connections to ES
 max-conns = 20
 # max numver of docs to keep in the BulkIndexer buffer