CI failure

[ianpartridge]

A recent CI job contained a failure: https://travis-ci.org/RuntimeTools/SwiftMetrics/jobs/490033860

Test Case '-[CoreSwiftMetricsTests.CoreSwiftMetricsTests testSwiftMetricsLatency]' started.
[Thu Feb  7 13:11:28 2019] com.ibm.diagnostics.healthcenter.loader INFO: Swift Application Metrics
2019-02-07 13:11:30.301 xctest[54720:79833] *** Assertion failure in -[XCTestExpectation fulfill], /Library/Caches/com.apple.xbs/Sources/XCTest/XCTest-14302.9/Sources/XCTestFramework/Async/XCTestExpectation.m:79
Test Case '-[CoreSwiftMetricsTests.CoreSwiftMetricsTests testSwiftMetricsLatency]' passed (2.148 seconds).
Test Case '-[CoreSwiftMetricsTests.CoreSwiftMetricsTests testSwiftMetricsLifecycle]' started.
2019-02-07 13:11:30.305 xctest[54720:79833] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'API violation - multiple calls made to -[XCTestExpectation fulfill] for Expect a latency event.'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fff39e0866b __exceptionPreprocess + 171
	1   libobjc.A.dylib                     0x00007fff61058942 objc_exception_throw + 48
	2   CoreFoundation                      0x00007fff39e0e252 +[NSException raise:format:arguments:] + 98
	3   Foundation                          0x00007fff3bf53340 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 193
	4   XCTest                              0x000000010030a724 -[XCTestExpectation fulfill] + 652
	5   SwiftMetricsPackageTests            0x00000001027f3fe5 $S21CoreSwiftMetricsTestsAAC04testbC7LatencyyyF07processF0L_7latencyy0bC00F4DataV_tF + 2277
	6   SwiftMetricsPackageTests            0x00000001027f4051 $S21CoreSwiftMetricsTestsAAC04testbC7LatencyyyF07processF0L_7latencyy0bC00F4DataV_tFTA + 17
	7   SwiftMetricsPackageTests            0x00000001029e66d7 $S12SwiftMetrics11LatencyDataVIegy_ACytIegnr_TR + 23
	8   SwiftMetricsPackageTests            0x00000001029e6731 $S12SwiftMetrics11LatencyDataVIegy_ACytIegnr_TRTA + 17
	9   SwiftMetricsPackageTests            0x00000001029e5afc $S12SwiftMetrics11LatencyDataVytIegnr_ACIegy_TR + 28
	10  SwiftMetricsPackageTests            0x00000001029e5b61 $S12SwiftMetrics11LatencyDataVytIegnr_ACIegy_TRTA + 17
	11  SwiftMetricsPackageTests            0x00000001029e5888 $S12SwiftMetrics0A7MonitorC12EventEmitterC7publish4datayAA11LatencyDataV_tFZ + 632
	12  SwiftMetricsPackageTests            0x00000001029ebfa3 $S12SwiftMetrics0A7MonitorC10raiseEvent4datayAA11LatencyDataV_tF + 435
	13  SwiftMetricsPackageTests            0x00000001029de659 $S12SwiftMetricsAAC8emitDatayyAA07LatencyD0VF + 169
	14  SwiftMetricsPackageTests            0x00000001029dab67 $S12SwiftMetricsAAC11testLatency33_70BA894F8E32C05E68B7F9F16B03EF2FLLyyFyycfU_yycfU_ + 711
	15  SwiftMetricsPackageTests            0x00000001029e2db3 $S12SwiftMetricsAAC11testLatency33_70BA894F8E32C05E68B7F9F16B03EF2FLLyyFyycfU_yycfU_TA + 19
	16  SwiftMetricsPackageTests            0x00000001029458fd $SIeg_IeyB_TR + 45
	17  libdispatch.dylib                   0x00007fff61c3d5fa _dispatch_call_block_and_release + 12
	18  libdispatch.dylib                   0x00007fff61c35db8 _dispatch_client_callout + 8
	19  libdispatch.dylib                   0x00007fff61c42cc5 _dispatch_queue_override_invoke + 880
	20  libdispatch.dylib                   0x00007fff61c379a9 _dispatch_root_queue_drain + 515
	21  libdispatch.dylib                   0x00007fff61c37755 _dispatch_worker_thread3 + 101
	22  libsystem_pthread.dylib             0x00007fff61f87169 _pthread_wqthread + 1387
	23  libsystem_pthread.dylib             0x00007fff61f86be9 start_wqthread + 13
)
libc++abi.dylib: terminating with uncaught exception of type NSException