Fix #676 Add an easier way to access Events API retry info

[seratch]

This pull request enables Events API listeners to access x-slack-retry-num / x-slack-retry-reason header values (retry_attempt / retry_reason in Socket Mode) in the following way.

app.event(AppMentionEvent.class, (req, ctx) -> {
  Integer num = ctx.getRetryNum(); // 0-3
  String reason = ctx.getRetryReason(); // timeout, http_error etc
  return ctx.ack();
});

Fixes #676

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-socket-mode (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[seratch]

@stevengill @mwbrooks
The names are a bit different here. Do we prefer retryAttempt? I personally feel retryNum is fine as it's a number.

[mwbrooks]

I'd personally prefer to keep the names consistent between the headers and variables. It makes it easier to understand that the two are identical and intended to match. So I'd lean toward retryNum as well.

[codecov]

Codecov Report

Merging #677 (8c6a5e5) into main (10165f6) will increase coverage by 0.06%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##               main     #677      +/-   ##
============================================
+ Coverage     78.79%   78.86%   +0.06%     
- Complexity     3052     3060       +8     
============================================
  Files           331      331              
  Lines          8830     8847      +17     
  Branches        833      839       +6     
============================================
+ Hits           6958     6977      +19     
+ Misses         1364     1363       -1     
+ Partials        508      507       -1     

Impacted Files	Coverage Δ	Complexity Δ
...m/slack/api/bolt/request/builtin/EventRequest.java	77.58% <83.33%> (+1.49%)	17.00 <0.00> (+4.00)
...t/socket_mode/request/SocketModeRequestParser.java	100.00% <100.00%> (ø)	6.00 <0.00> (+2.00)
...i/methods/metrics/impl/MemoryMetricsDatastore.java	92.95% <0.00%> (+1.40%)	42.00% <0.00%> (+1.00%)
.../com/slack/api/bolt/socket_mode/SocketModeApp.java	81.66% <0.00%> (+3.33%)	13.00% <0.00%> (+1.00%)

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[mwbrooks]

I'd personally prefer to keep the names consistent between the headers and variables. It makes it easier to understand that the two are identical and intended to match. So I'd lean toward retryNum as well.

[sagar-ranglani]

Any plans on adding this for boltjs?