Flaky test TestTBufferedServer_Metrics

[yurishkuro]

it strikes again https://travis-ci.com/github/yurishkuro/jaeger/jobs/440719322#L679

=== RUN   TestTBufferedServer_Metrics
    metricstest.go:49: 
        	Error Trace:	metricstest.go:49
        	            				metricstest.go:37
        	            				tbuffered_server_test.go:156
        	Error:      	Not equal: 
        	            	expected: int(1)
        	            	actual  : int64(0)
        	Test:       	TestTBufferedServer_Metrics
        	Messages:   	expected metric name: thrift.udp.server.read.errors, tags: map[]
--- FAIL: TestTBufferedServer_Metrics (0.01s)
FAIL
coverage: 97.3% of statements
FAIL	github.com/jaegertracing/jaeger/cmd/agent/app/servers	0.051s