Flaky `TestAppenderRetryDocument` unit test

[rubvs]

This happens on very rare occasions in CI.

=== RUN   TestAppenderRetryDocument/nocompression
    appender_test.go:1331: 
        	Error Trace:	/home/runner/work/go-docappender/go-docappender/appender_test.go:1331
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0
        	Test:       	TestAppenderRetryDocument/nocompression
    docappendertest.go:241: 
        	Error Trace:	/home/runner/work/go-docappender/go-docappender/docappendertest/docappendertest.go:241
        	            				/home/runner/work/go-docappender/go-docappender/appender_test.go:1346
        	            				/home/runner/work/go-docappender/go-docappender/docappendertest/docappendertest.go:251
        	            				/home/runner/work/go-docappender/go-docappender/appender_test.go:1343
        	Error:      	Not equal: 
        	            	expected: attribute.Set{equivalent:attribute.Distinct{iface:[1]attribute.KeyValue{attribute.KeyValue{Key:"greatest_retry", Value:attribute.Value{vtype:2, numeric:0x2, stringly:"", slice:interface {}(nil)}}}}}
        	            	actual  : attribute.Set{equivalent:attribute.Distinct{iface:[1]attribute.KeyValue{attribute.KeyValue{Key:"greatest_retry", Value:attribute.Value{vtype:2, numeric:0x1, stringly:"", slice:interface {}(nil)}}}}}
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -7,3 +7,3 @@
        	            	      vtype: (attribute.Type) 2,
        	            	-     numeric: (uint64) 2,
        	            	+     numeric: (uint64) 1,
        	            	      stringly: (string) "",
        	Test:       	TestAppenderRetryDocument/nocompression
    appender_test.go:1351: 
        	Error Trace:	/home/runner/work/go-docappender/go-docappender/appender_test.go:1351
        	Error:      	Not equal: 
        	            	expected: 2
        	            	actual  : 1
        	Test:       	TestAppenderRetryDocument/nocompression
=== RUN   TestAppenderRetryDocument/gzip