[receiver/discovery] Remove first_only field from the matching rule

[dmitryax]

This is another step towards moving from discovery events to the entity events. The entity events will be emitted once a new matching rule matches a metric or log statement. After the initial emit the same entity state will be emitted on a regular basis, not when we get another match. This makes the first_only config option conflicting with the way how the entity events are expected to be emitted.

WIth this particular change, we switch behavior to emit_first=true by design. Given that all the discovery rules have the same value preconfigured, it's not a breaking change for users who rely on them. It is a breaking change for users who have custom discovery rules with emit_first=false. This is acceptable given the "In development" status of the receiver.

[atoulme]

=== RUN   TestStatementEvaluation/strict/append_true/successful
panic: test timed out after 3m0s
running tests:
	TestStatementEvaluation (3m0s)
	TestStatementEvaluation/strict (3m0s)
	TestStatementEvaluation/strict/append_true (3m0s)
	TestStatementEvaluation/strict/append_true/successful (3m0s)

goroutine 21 [running]:
testing.(*M).startAlarm.func1()
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:2259 +0x259
created by time.goFunc
	/opt/hostedtoolcache/go/1.21.5/x64/src/time/sleep.go:176 +0x45

goroutine 1 [chan receive, 2 minutes]:
testing.(*T).Run(0xc0001029c0, {0x1315ea0, 0x17}, 0x135a9d8)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1649 +0x871
testing.runTests.func1(0x0?)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:2054 +0x85
testing.tRunner(0xc0001029c0, 0xc0001f7af8)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0x262
testing.runTests(0xc00013[918](https://github.com/signalfx/splunk-otel-collector/actions/runs/8547750812/job/23420441724?pr=4593#step:4:919)0?, {0x1ae3ce0, 0x1a, 0x1a}, {0x1c?, 0x1e?, 0x1af1600?})
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:2052 +0x8ae
testing.(*M).Run(0xc000139180)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1925 +0xcd8
main.main()
	_testmain.go:99 +0x2be

goroutine 123 [chan receive, 2 minutes]:
testing.(*T).Run(0xc00045da00, {0x130aa48, 0xa}, 0xc000115338)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1649 +0x871
github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation.func1.1(0x0?)
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:56 +0x309
testing.tRunner(0xc00045da00, 0xc0001152f0)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 122
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1648 +0x846

goroutine 122 [chan receive, 2 minutes]:
testing.(*T).Run(0xc00045d6c0, {0xc000379435, 0xb}, 0xc0001152f0)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1649 +0x871
github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation.func1(0x0?)
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:45 +0x7f
testing.tRunner(0xc00045d6c0, 0xc0002a9c30)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 121
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1648 +0x846

goroutine 121 [chan receive, 2 minutes]:
testing.(*T).Run(0xc00045d520, {0x1306339, 0x6}, 0xc0002a9c30)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1649 +0x871
github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation(0x0?)
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:43 +0xa5
testing.tRunner(0xc00045d520, 0x135a9d8)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 1
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1648 +0x846

goroutine 124 [semacquire, 2 minutes]:
sync.runtime_Semacquire(0xc00037[950](https://github.com/signalfx/splunk-otel-collector/actions/runs/8547750812/job/23420441724?pr=4593#step:4:951)8?)
	/opt/hostedtoolcache/go/1.21.5/x64/src/runtime/sema.go:62 +0x25
sync.(*WaitGroup).Wait(0xc000379500)
	/opt/hostedtoolcache/go/1.21.5/x64/src/sync/waitgroup.go:116 +0xa5
github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation.func1.1.1(0x0?)
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:96 +0xf38
testing.tRunner(0xc00045dd40, 0xc000115338)
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 123
	/opt/hostedtoolcache/go/1.21.5/x64/src/testing/testing.go:1648 +0x846

goroutine 125 [chan receive, 2 minutes]:
github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation.func1.1.1.1()
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:93 +0x4c
created by github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver.TestStatementEvaluation.func1.1.1 in goroutine 124
	/home/runner/work/splunk-otel-collector/splunk-otel-collector/internal/receiver/discoveryreceiver/statement_evaluator_test.go:92 +0xf2b
FAIL	github.com/signalfx/splunk-otel-collector/internal/receiver/discoveryreceiver	180.019s