Modernise OpenTelemetry Collector example config (#3518) (#3525)

* Use OpenTelemetry Collector `debug` exporter

The `logging` exporter is deprecated, and its
recommended replacement is the `debug` exporter.

* Use upstream-documented env var expansion syntax

See
https://opentelemetry.io/docs/collector/configuration/#environment-variables

* Update docs link

* More fixes

Noticed a couple of unrelated issues while previewing changes:
 - broken links to OTLP and Elasticsearch exporters
 - unnecessary parentheses around OTLP/gRPC and OTLP/HTTP

(cherry picked from commit 1debccc)

Co-authored-by: Andrew Wilkins <[email protected]>

docs/en/observability/apm/otel-direct.asciidoc

@@ -32,31 +32,31 @@ processors: <2>
   batch:
 
 exporters:
-  logging:
-    loglevel: warn <3>
+  debug:
+    verbosity: detailed <3>
   otlp/elastic: <4>
     # Elastic APM server https endpoint without the "https://" prefix
-    endpoint: "${ELASTIC_APM_SERVER_ENDPOINT}" <5> <7>
+    endpoint: "${env:ELASTIC_APM_SERVER_ENDPOINT}" <5> <7>
     headers:
       # Elastic APM Server secret token
-      Authorization: "Bearer ${ELASTIC_APM_SECRET_TOKEN}" <6> <7>
+      Authorization: "Bearer ${env:ELASTIC_APM_SECRET_TOKEN}" <6> <7>
 
 service:
   pipelines:
     traces:
       receivers: [otlp]
-      exporters: [logging, otlp/elastic]
+      exporters: [debug, otlp/elastic]
     metrics:
       receivers: [otlp]
-      exporters: [logging, otlp/elastic]
+      exporters: [debug, otlp/elastic]
     logs: <8>
       receivers: [otlp]
-      exporters: [logging, otlp/elastic]
+      exporters: [debug, otlp/elastic]
 ----
 <1> The receivers, like the
 https://github.com/open-telemetry/opentelemetry-collector/tree/main/receiver/otlpreceiver[OTLP receiver], that forward data emitted by APM agents, or the https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/hostmetricsreceiver[host metrics receiver].
 <2> We recommend using the https://github.com/open-telemetry/opentelemetry-collector/blob/main/processor/batchprocessor/README.md[Batch processor] and the https://github.com/open-telemetry/opentelemetry-collector/blob/main/processor/memorylimiterprocessor/README.md[memory limiter processor]. For more information, see https://github.com/open-telemetry/opentelemetry-collector/blob/main/processor/README.md#recommended-processors[recommended processors].
-<3> The https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/loggingexporter[logging exporter] is helpful for troubleshooting and supports various logging levels, like `debug`, `info`, `warn`, and `error`.
+<3> The https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/debugexporter[debug exporter] is helpful for troubleshooting, and supports configurable verbosity levels: `basic` (default), `normal`, and `detailed`.
 <4> Elastic {observability} endpoint configuration.
 APM Server supports a ProtoBuf payload via both the OTLP protocol over gRPC transport {ot-grpc}[(OTLP/gRPC)]
 and the OTLP protocol over HTTP transport {ot-http}[(OTLP/HTTP)].
@@ -65,13 +65,13 @@ https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/otl
 https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/otlpexporter[OTLP/gRPC exporter].
 <5> Hostname and port of the APM Server endpoint. For example, `elastic-apm-server:8200`.
 <6> Credential for Elastic APM <<secret-token,secret token authorization>> (`Authorization: "Bearer a_secret_token"`) or <<api-key,API key authorization>> (`Authorization: "ApiKey an_api_key"`).
-<7> Environment-specific configuration parameters can be conveniently passed in as environment variables documented https://opentelemetry.io/docs/collector/configuration/#configuration-environment-variables[here] (e.g. `ELASTIC_APM_SERVER_ENDPOINT` and `ELASTIC_APM_SECRET_TOKEN`).
+<7> Environment-specific configuration parameters can be conveniently passed in as environment variables documented https://opentelemetry.io/docs/collector/configuration/#environment-variables[here] (e.g. `ELASTIC_APM_SERVER_ENDPOINT` and `ELASTIC_APM_SECRET_TOKEN`).
 <8>  preview:[] To send OpenTelemetry logs to {stack} version 8.0+, declare a `logs` pipeline.
 
 You're now ready to export traces and metrics from your services and applications.
 
-TIP: When using the OpenTelemetry collector, you should always prefer sending data via the [`OTLP` exporter](https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/otlphttpexporter) to an Elastic APM Server.
-Other methods, like using the [`elasticsearch` exporter](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/elasticsearchexporter) to send data directly to {es} will send data to the {stack},
+TIP: When using the OpenTelemetry collector, you should always prefer sending data via the https://github.com/open-telemetry/opentelemetry-collector/tree/main/exporter/otlphttpexporter[`OTLP` exporter] to an Elastic APM Server.
+Other methods, like using the https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/elasticsearchexporter[`elasticsearch` exporter] to send data directly to {es} will send data to the {stack},
 but will bypass all of the validation and data processing that the APM Server performs.
 In addition, your data will not be viewable in the {kib} {observability} apps if you use the `elasticsearch` exporter.
 
@@ -128,7 +128,7 @@ and <<open-telemetry-visualize,visualizing metrics>> in {kib}.
 [[open-telemetry-proxy-apm]]
 ==== Proxy requests to APM Server
 
-APM Server supports both the {ot-grpc}[(OTLP/gRPC)] and {ot-http}[(OTLP/HTTP)] protocol on the same port as Elastic APM agent requests. For ease of setup, we recommend using OTLP/HTTP when proxying or load balancing requests to the APM Server.
+APM Server supports both the {ot-grpc}[OTLP/gRPC] and {ot-http}[OTLP/HTTP] protocol on the same port as Elastic APM agent requests. For ease of setup, we recommend using OTLP/HTTP when proxying or load balancing requests to the APM Server.
 
 If you use the OTLP/gRPC protocol, requests to the APM Server must use either HTTP/2 over TLS or HTTP/2 Cleartext (H2C). No matter which protocol is used, OTLP/gRPC requests will have the header: `"Content-Type: application/grpc"`.