Fix goconst issues

See,
$ gometalinter --vendor --disable-all --enable=goconst --disable=gotype --deadline=6m ./...
build.go:113:15⚠️ 2 other occurrence(s) of "linux" found in: build.go:119:15 build.go:491:34 (goconst)
build.go:119:15⚠️ 2 other occurrence(s) of "linux" found in: build.go:113:15 build.go:491:34 (goconst)
build.go:491:34⚠️ 2 other occurrence(s) of "linux" found in: build.go:113:15 build.go:119:15 (goconst)
build.go:381:21⚠️ 2 other occurrence(s) of "windows" found in: build.go:423:13 build.go:487:13 (goconst)
build.go:423:13⚠️ 2 other occurrence(s) of "windows" found in: build.go:381:21 build.go:487:13 (goconst)
build.go:487:13⚠️ 2 other occurrence(s) of "windows" found in: build.go:381:21 build.go:423:13 (goconst)
pkg/api/dashboard.go:67:22⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:67:35⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:131:10⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/dashboard.go:406:13⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/folder.go:98:22 pkg/api/folder.go:98:35 (goconst)
pkg/api/folder.go:98:22⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:35 (goconst)
pkg/api/folder.go:98:35⚠️ 5 other occurrence(s) of "Anonymous" found in: pkg/api/dashboard.go:67:22 pkg/api/dashboard.go:67:35 pkg/api/dashboard.go:131:10 pkg/api/dashboard.go:406:13 pkg/api/folder.go:98:22 (goconst)
pkg/api/index.go:63:47⚠️ 2 other occurrence(s) of "light" found
in: pkg/api/index.go:91:22 pkg/api/index.go:93:16 (goconst)
pkg/api/index.go:91:22⚠️ 2 other occurrence(s) of "light" found in: pkg/api/index.go:63:47 pkg/api/index.go:93:16 (goconst)
pkg/api/index.go:93:16⚠️ 2 other occurrence(s) of "light" found in: pkg/api/index.go:63:47 pkg/api/index.go:91:22 (goconst)
pkg/components/null/float.go:71:25⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:103:10 pkg/components/null/float.go:112:10 (goconst)
pkg/components/null/float.go:103:10⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:71:25 pkg/components/null/float.go:112:10 (goconst)
pkg/components/null/float.go:112:10⚠️ 2 other occurrence(s) of "null" found in: pkg/components/null/float.go:71:25 pkg/components/null/float.go:103:10 (goconst)
pkg/services/alerting/notifiers/pagerduty.go:79:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/kafka.go:64:16 pkg/services/alerting/notifiers/opsgenie.go:98:16 (goconst)
pkg/services/alerting/notifiers/kafka.go:64:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/pagerduty.go:79:16 pkg/services/alerting/notifiers/opsgenie.go:98:16 (goconst)
pkg/services/alerting/notifiers/opsgenie.go:98:16⚠️ 2 other occurrence(s) of "Triggered metrics:\n\n" found in: pkg/services/alerting/notifiers/pagerduty.go:79:16 pkg/services/alerting/notifiers/kafka.go:64:16 (goconst)
pkg/social/social.go:85:11⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:162:14 pkg/social/social.go:197:11 (goconst)
pkg/social/social.go:162:14⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:85:11 pkg/social/social.go:197:11 (goconst)
pkg/social/social.go:197:11⚠️ 2 other occurrence(s) of "grafana_com" found in: pkg/social/social.go:85:11 pkg/social/social.go:162:14 (goconst)
pkg/tsdb/elasticsearch/time_series_query.go:92:17⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:167:31 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:152:8⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:167:31 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:167:31⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:315:9 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:315:9⚠️ 3 other occurrence(s) of "count" found in: pkg/tsdb/elasticsearch/time_series_query.go:92:17 pkg/tsdb/elasticsearch/response_parser.go:152:8 pkg/tsdb/elasticsearch/response_parser.go:167:31 (goconst)
pkg/tsdb/elasticsearch/time_series_query.go:78:9⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/response_parser.go:84:22 pkg/tsdb/elasticsearch/response_parser.go:369:24 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:84:22⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/time_series_query.go:78:9 pkg/tsdb/elasticsearch/response_parser.go:369:24 (goconst)
pkg/tsdb/elasticsearch/response_parser.go:369:24⚠️ 2 other occurrence(s) of "date_histogram" found in: pkg/tsdb/elasticsearch/time_series_query.go:78:9 pkg/tsdb/elasticsearch/response_parser.go:84:22 (goconst)

pkg/components/null/float.go

@@ -8,6 +8,10 @@ import (
 	"strconv"
 )
 
+const (
+	nullString = "null"
+)
+
 // Float is a nullable float64.
 // It does not consider zero values to be null.
 // It will decode to null, not zero, if null.
@@ -68,7 +72,7 @@ func (f *Float) UnmarshalJSON(data []byte) error {
 // It will return an error if the input is not an integer, blank, or "null".
 func (f *Float) UnmarshalText(text []byte) error {
 	str := string(text)
-	if str == "" || str == "null" {
+	if str == "" || str == nullString {
 		f.Valid = false
 		return nil
 	}
@@ -82,7 +86,7 @@ func (f *Float) UnmarshalText(text []byte) error {
 // It will encode null if this Float is null.
 func (f Float) MarshalJSON() ([]byte, error) {
 	if !f.Valid {
-		return []byte("null"), nil
+		return []byte(nullString), nil
 	}
 	return []byte(strconv.FormatFloat(f.Float64, 'f', -1, 64)), nil
 }
@@ -100,7 +104,7 @@ func (f Float) MarshalText() ([]byte, error) {
 // It will encode a blank string if this Float is null.
 func (f Float) String() string {
 	if !f.Valid {
-		return "null"
+		return nullString
 	}
 
 	return fmt.Sprintf("%1.3f", f.Float64)
@@ -109,7 +113,7 @@ func (f Float) String() string {
 // FullString returns float as string in full precision
 func (f Float) FullString() string {
 	if !f.Valid {
-		return "null"
+		return nullString
 	}
 
 	return fmt.Sprintf("%f", f.Float64)