prom: Save __name__ before relabel for look up metric type

[pingleig]

Description of the issue

Fix #190

Description of changes

Save name before relabel using relabel.

License

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Tests

• manually
• unit test
• internal integ test

[pingleig]

oops, it seems this breaks unit test

=== RUN   TestNewMetricsTypeHandler_HandleWithNormalTarget
2021/03/22 22:11:00 D! metric name changed from "" to "m3" during relabel
2021/03/22 22:11:00 E! metricsHandler NO metaData for m3 | instance1 | job1 
2021/03/22 22:11:00 E! metric_type ERROR: m3|job1|instance1|  
2021/03/22 22:11:00 D! metric name changed from "" to "m1" during relabel
2021/03/22 22:11:00 E! metricsHandler NO metaData for m1 | instance1 | job1 
2021/03/22 22:11:00 E! metric_type ERROR: m1|job1|instance1|  
2021/03/22 22:11:00 D! metric name changed from "" to "m2" during relabel
2021/03/22 22:11:00 E! metricsHandler NO metaData for m2 | instance1 | job1 
2021/03/22 22:11:00 E! metric_type ERROR: m2|job1|instance1|  
    metric_type_handler_test.go:220: 
        	Error Trace:	metric_type_handler_test.go:220
        	Error:      	Not equal: 
        	            	expected: 2
        	            	actual  : 0
        	Test:       	TestNewMetricsTypeHandler_HandleWithNormalTarget
--- FAIL: TestNewMetricsTypeHandler_HandleWithNormalTarget (0.00s)
panic: runtime error: index out of range [0] with length 0 [recovered]
	panic: runtime error: index out of range [0] with length 0

goroutine 58 [running]:
testing.tRunner.func1.2(0x2bf1c80, 0xc0001a2180)
	/Users/runner/hostedtoolcache/go/1.16.2/x64/src/testing/testing.go:1144 +0x332
testing.tRunner.func1(0xc00023e780)
	/Users/runner/hostedtoolcache/go/1.16.2/x64/src/testing/testing.go:1147 +0x4b6
panic(0x2bf1c80, 0xc0001a2180)
	/Users/runner/hostedtoolcache/go/1.16.2/x64/src/runtime/panic.go:965 +0x1b9
github.com/aws/amazon-cloudwatch-agent/plugins/inputs/prometheus_scraper.TestNewMetricsTypeHandler_HandleWithNormalTarget(0xc00023e780)
	/Users/runner/work/amazon-cloudwatch-agent/amazon-cloudwatch-agent/amazon-cloudwatch-agent/plugins/inputs/prometheus_scraper/metric_type_handler_test.go:231 +0xaec
testing.tRunner(0xc00023e780, 0x2e2e730)
	/Users/runner/hostedtoolcache/go/1.16.2/x64/src/testing/testing.go:1194 +0xef
created by testing.(*T).Run
	/Users/runner/hostedtoolcache/go/1.16.2/x64/src/testing/testing.go:1239 +0x2b3
FAIL	github.com/aws/amazon-cloudwatch-agent/plugins/inputs/prometheus_scraper	0.469s