Fix race in user agent test

[dashpole]

Failing run: https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/actions/runs/3887696124/jobs/6634249603

=== RUN   TestExportMetricsWithUserAgent
=== RUN   TestExportMetricsWithUserAgent/default
=== RUN   TestExportMetricsWithUserAgent/override_user_agent
panic: Fail in goroutine after TestExportMetricsWithUserAgent/default has completed

goroutine 131 [running]:
testing.(*common).Fail(0xc0002d4ea0)
	/opt/hostedtoolcache/go/1.18.9/x64/src/testing/testing.go:817 +0xe5
testing.(*common).Errorf(0xc0002d4ea0, {0xc21425?, 0xc0002d4ea0?}, {0xc000318000?, 0x47554f?, 0x11f1bc0?})
	/opt/hostedtoolcache/go/1.18.9/x64/src/testing/testing.go:934 +0x65
github.com/stretchr/testify/assert.Fail({0xd3ba00, 0xc0002d4ea0}, {0xc0004dc310, 0x6f}, {0x0, 0x0, 0x0})
	/home/runner/go/pkg/mod/github.com/stretchr/testify@v1.8.1/assert/assertions.go:264 +0x384
github.com/stretchr/testify/assert.Regexp({0xd3ba00, 0xc0002d4ea0}, {0xaea680, 0xc0002dbb80}, {0xaea680, 0xc0002dbb90}, {0x0, 0x0, 0x0})
	/home/runner/go/pkg/mod/github.com/stretchr/testify@v1.8.1/assert/assertions.go:1485 +0x13e
github.com/stretchr/testify/require.Regexp({0xd3e7d0, 0xc0002d4ea0}, {0xaea680, 0xc0002dbb80}, {0xaea680, 0xc0002dbb90}, {0x0, 0x0, 0x0})
	/home/runner/go/pkg/mod/github.com/stretchr/testify@v1.8.1/require/require.go:1735 +0xc5
github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric.TestExportMetricsWithUserAgent.func1.3()
	/home/runner/work/opentelemetry-operations-go/opentelemetry-operations-go/exporter/metric/metric_test.go:890 +0x85
created by github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric.TestExportMetricsWithUserAgent.func1
	/home/runner/work/opentelemetry-operations-go/opentelemetry-operations-go/exporter/metric/metric_test.go:887 +0x20a
FAIL	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/metric	0.045s
FAIL
make: *** [Makefile:134: test] Error 1

[codecov]

Codecov Report

Merging #568 (8cededf) into main (771e067) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #568   +/-   ##
=======================================
  Coverage   64.06%   64.06%           
=======================================
  Files          37       37           
  Lines        4182     4182           
=======================================
  Hits         2679     2679           
  Misses       1390     1390           
  Partials      113      113           

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[dashpole]

(cla/google seems to be stuck)