Datadog: restore "resource.name" tag

[dgoffredo]

These changes restore the Dataodog tracer's ability to set the Datadog notion of span "resource name" by setting a special tag named "resource.name".

This was previously possible with the OpenTracing-based implementation of Datadog tracing, but I removed the capability when I moved Envoy's Datadog extension away from OpenTracing.

It turns out that customers were using this feature. This pull request restores it.

Please consider backporting this revision as a bug fix once it is merged.

Testing

I modified some existing unit tests.

Also, by applying the patch below, I used the Datadog "demo" with a local build of Envoy to verify that a request_header-based custom tag for the tag "resource.name" really does result in a change to the resource name:

diff --git a/source/extensions/tracers/datadog/demo/envoy.yaml b/source/extensions/tracers/datadog/demo/envoy.yaml
index 839ba18aac..305c76059f 100644
--- a/source/extensions/tracers/datadog/demo/envoy.yaml
+++ b/source/extensions/tracers/datadog/demo/envoy.yaml
@@ -18,6 +18,13 @@ static_resources:
                 "@type": type.googleapis.com/envoy.config.trace.v3.DatadogConfig
                 collector_cluster: datadog_agent
                 service_name: envoy-demo
+            custom_tags:
+            - tag: "header.thing"
+              request_header:
+                name: "x-thing"
+            - tag: "resource.name"
+              request_header:
+                name: "x-datadog-resource"
           access_log:
           - name: envoy.access_loggers.file
             typed_config:

---

Commit Message: restore ability to set resource name by tag in Datadog tracer
Additional Description:
Risk Level: low
Testing: See modified unit tests. Also performed manual integration testing with the Datadog "demo."
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

[dgoffredo]

Thanks @mattklein123!

Please consider backporting this revision as a bug fix once it is merged.

Can we get this tagged for review?

[hiddewie]

Hi @mattklein123 and @dgoffredo. Thanks for picking this PR up. We observed that it was not part of the recent 1.28 release. Is there a plan to backport this fix into previous Envoy versions?

[dgoffredo]

I would like for this PR to be reviewed for backporting as a bug fix onto v1.27 and v1.28. @mattklein123, how do I start that process?