Update APM server code for breaking changes in libbeat API (ES client)

[ycombinator]

We're undertaking some major refactoring/cleanup in the libbeat codebase, specifically in the Elasticsearch output package. The PR for this is elastic/beats#16150.

Please see the "Developer Docs" section in the PR description for how it might impact APM server code. From my snooping around, it looks like the following locations might be impacted:

• apm-server/beater/beater.go

  Line 210 in 72c446d

  esClient, err := elasticsearch.NewConnectedClient(esConfig)

• apm-server/beater/beater.go

  Line 217 in 72c446d

  _, err := elasticsearch.RegisterConnectCallback(func(esClient *elasticsearch.Client) error {

• apm-server/_beats/libbeat/scripts/cmd/stress_pipeline/main.go

  Line 32 in eaaf95c

  _ "github.com/elastic/beats/libbeat/outputs/elasticsearch"

• apm-server/elasticsearch/config_test.go

  Lines 102 to 105 in 6ef9305

  	libbeatType, _ := reflect2.TypeByPackageName(
  	"github.com/elastic/apm-server/vendor/github.com/elastic/beats/libbeat/outputs/elasticsearch",
  	"elasticsearchConfig",
  	).(reflect2.StructType)

• apm-server/ingest/pipeline/register.go

  Line 31 in 3732dad

  func RegisterPipelines(esClient *es.Client, overwrite bool, path string) error {

• apm-server/ingest/pipeline/register_test.go

  Line 34 in 3732dad

  esClients, err := es.NewElasticsearchClients(getFakeESConfig(9200))

• apm-server/ingest/pipeline/register_test.go

  Line 52 in 3732dad

  invalidClients, err := es.NewElasticsearchClients(getFakeESConfig(1234))

[axw]

Assigning myself, since this is going to get tangled up with go-modules migration.

[ycombinator]

@axw you might want to be aware of #3416 as well — these two are related.

[axw]

👍 thanks @ycombinator