[chore][pkg/resourcetotelemetry] reduce memory usage by call ensure capacity

[philchia]

Description:

1. Reduce memory usage by ensure capacity before merge two maps

Link to tracking Issue:

N/A

Testing:

N/A

Documentation:

N/A

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: philchia / name: Phil (546addd)

[philchia]

Here is the benchmark result:

Before:

$ go test -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/pkg/resourcetotelemetry
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkJoinAttributes/merge_10_into_10-12               341402              3347 ns/op            1628 B/op         34 allocs/op
BenchmarkJoinAttributes/merge_10_into_20-12               180139              7004 ns/op            3229 B/op         64 allocs/op
BenchmarkJoinAttributes/merge_20_into_10-12               268239              4496 ns/op            2909 B/op         35 allocs/op
BenchmarkJoinAttributes/merge_30_into_10-12               198063              5812 ns/op            2909 B/op         35 allocs/op
BenchmarkJoinAttributes/merge_10_into_30-12               115959             10164 ns/op            5022 B/op         94 allocs/op
PASS
ok      github.com/open-telemetry/opentelemetry-collector-contrib/pkg/resourcetotelemetry       7.237s

After:

$ go test -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/pkg/resourcetotelemetry
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkJoinAttributes/merge_10_into_10-12               340310              3304 ns/op            1628 B/op         34 allocs/op
BenchmarkJoinAttributes/merge_10_into_20-12               186794              6491 ns/op            2973 B/op         64 allocs/op
BenchmarkJoinAttributes/merge_20_into_10-12               276632              4272 ns/op            2013 B/op         34 allocs/op
BenchmarkJoinAttributes/merge_30_into_10-12               213165              5562 ns/op            2269 B/op         34 allocs/op
BenchmarkJoinAttributes/merge_10_into_30-12               119604             10185 ns/op            4254 B/op         94 allocs/op
PASS
ok      github.com/open-telemetry/opentelemetry-collector-contrib/pkg/resourcetotelemetry       7.138s
(base)