feat: [RMS-1524] add DeadlineExceeded status code (#416)

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<!-- A short description of what your PR does and what it solves. -->
## What this PR does / why we need it

This PR is adding `DeadlineExceeded` status code. It's error type that
is quite different to other server side errors and we would like to
track and measure it separately (in our dashboards etc.).

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## Jira ID

[RMS-1524]

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## Notes for your reviewers



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[RMS-1524]:
https://outreach-io.atlassian.net/browse/RMS-1524?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

pkg/statuscodes/statuscodes.go

@@ -68,6 +68,9 @@ const (
 	// Deprecated: In reality server-side errors should fall into one of the above 3 errors, and this inclusion was
 	// a mistake.  It's not worth a breaking change to revoke at this time, though, so it shall live on.
 	UnknownError StatusCode = 803
+	// DeadlineExceeded is for when the server is unable to complete the request within the configured deadline and the
+	// request times out. This error is retriable, but the duration for backoff is unknown.
+	DeadlineExceeded StatusCode = 804
 )
 
 //go:generate ../../scripts/shell-wrapper.sh gobin.sh golang.org/x/tools/cmd/[email protected] -type=StatusCode
@@ -136,6 +139,8 @@ func FromString(s string) (StatusCode, bool) {
 		return Unavailable, true
 	case UnknownError.String():
 		return UnknownError, true
+	case DeadlineExceeded.String():
+		return DeadlineExceeded, true
 	default:
 		return UnknownError, false
 	}

pkg/statuscodes/statuscodes_test.go

@@ -16,6 +16,7 @@ func TestStatusCodeUnmarshalText(t *testing.T) {
 		NotImplemented,
 		Unavailable,
 		UnknownError,
+		DeadlineExceeded,
 	}
 
 	for _, code := range codes {