Don't use json.Number to encode/decode events event_ts

This field should always be stored/used as a string according to [1].
This also makes the type more consistent across events.

[1] slackhq/slack-api-docs#7 (comment)

slackevents/inner_events.go

@@ -3,8 +3,6 @@
 package slackevents
 
 import (
-	"encoding/json"
-
 	"github.com/slack-go/slack"
 )
 
@@ -16,13 +14,13 @@ type EventsAPIInnerEvent struct {
 
 // AppMentionEvent is an (inner) EventsAPI subscribable event.
 type AppMentionEvent struct {
-	Type            string      `json:"type"`
-	User            string      `json:"user"`
-	Text            string      `json:"text"`
-	TimeStamp       string      `json:"ts"`
-	ThreadTimeStamp string      `json:"thread_ts"`
-	Channel         string      `json:"channel"`
-	EventTimeStamp  json.Number `json:"event_ts"`
+	Type            string `json:"type"`
+	User            string `json:"user"`
+	Text            string `json:"text"`
+	TimeStamp       string `json:"ts"`
+	ThreadTimeStamp string `json:"thread_ts"`
+	Channel         string `json:"channel"`
+	EventTimeStamp  string `json:"event_ts"`
 
 	// When Message comes from a channel that is shared between workspaces
 	UserTeam   string `json:"user_team,omitempty"`
@@ -34,12 +32,12 @@ type AppMentionEvent struct {
 
 // AppHomeOpenedEvent Your Slack app home was opened.
 type AppHomeOpenedEvent struct {
-	Type           string      `json:"type"`
-	User           string      `json:"user"`
-	Channel        string      `json:"channel"`
-	EventTimeStamp json.Number `json:"event_ts"`
-	Tab            string      `json:"tab"`
-	View           slack.View  `json:"view"`
+	Type           string     `json:"type"`
+	User           string     `json:"user"`
+	Channel        string     `json:"channel"`
+	EventTimeStamp string     `json:"event_ts"`
+	Tab            string     `json:"tab"`
+	View           slack.View `json:"view"`
 }
 
 // AppUninstalledEvent Your Slack app was uninstalled.
@@ -198,15 +196,15 @@ type sharedLinks struct {
 // TODO: Improve this so that it is not required to manually parse ChannelType
 type MessageEvent struct {
 	// Basic Message Event - https://api.slack.com/events/message
-	ClientMsgID     string      `json:"client_msg_id"`
-	Type            string      `json:"type"`
-	User            string      `json:"user"`
-	Text            string      `json:"text"`
-	ThreadTimeStamp string      `json:"thread_ts"`
-	TimeStamp       string      `json:"ts"`
-	Channel         string      `json:"channel"`
-	ChannelType     string      `json:"channel_type"`
-	EventTimeStamp  json.Number `json:"event_ts"`
+	ClientMsgID     string `json:"client_msg_id"`
+	Type            string `json:"type"`
+	User            string `json:"user"`
+	Text            string `json:"text"`
+	ThreadTimeStamp string `json:"thread_ts"`
+	TimeStamp       string `json:"ts"`
+	Channel         string `json:"channel"`
+	ChannelType     string `json:"channel_type"`
+	EventTimeStamp  string `json:"event_ts"`
 
 	// When Message comes from a channel that is shared between workspaces
 	UserTeam   string `json:"user_team,omitempty"`
@@ -308,9 +306,9 @@ type TokensRevokedEvent struct {
 
 // EmojiChangedEvent is the event of custom emoji has been added or changed
 type EmojiChangedEvent struct {
-	Type           string      `json:"type"`
-	Subtype        string      `json:"subtype"`
-	EventTimeStamp json.Number `json:"event_ts"`
+	Type           string `json:"type"`
+	Subtype        string `json:"subtype"`
+	EventTimeStamp string `json:"event_ts"`
 
 	// filled out when custom emoji added
 	Name string `json:"name,omitempty"`

socketmode/socketmode_test.go

@@ -236,7 +236,7 @@ func TestEventParsing(t *testing.T) {
 						TimeStamp:       "1610927831.000200",
 						ThreadTimeStamp: "",
 						Channel:         "redacted",
-						EventTimeStamp:  json.Number("1610927831.000200"),
+						EventTimeStamp:  "1610927831.000200",
 					},
 				},
 			},