fix(microbenchmarks): restore CI Visibility reporting to Datadog (#8483)

## Summary of changes

- Set `DD_CIVISIBILITY_AGENTLESS_ENABLED=1` on the benchmark process in
`.gitlab/benchmarks/microbenchmarks/scripts/run-benchmarks.ps1` when
`DD_API_KEY` is present, and default `DD_SITE=datadoghq.com`.
- Change the SSM parameter name in
`.gitlab/benchmarks/microbenchmarks.yml` from
`ci.dd-trace-dotnet.dd_api_key` to `ci.dd-trace-dotnet.dd_api_key-prod`
to match the key the previous pipeline was using.

## Reason for change

Microbenchmark results stopped being reported to Datadog on April 8,
2026, after #8300 moved the Windows microbenchmark runner from
`benchmarking-platform@dd-trace-dotnet/micro` into this repo. The
`Datadog.Trace.BenchmarkDotNet` exporter ships results to Datadog via CI
Visibility, and the old flow enabled agentless mode
(`DD_CIVISIBILITY_AGENTLESS_ENABLED=1`) with a production SSM key — two
details that were not carried over in the rewrite, so the exporter
silently failed to reach intake.

Cross-reference with the old script:
`benchmarking-platform/run-benchmarks.ps1:128`
(`$env:DD_CIVISIBILITY_AGENTLESS_ENABLED=1`) and
`benchmarking-platform/steps/run-windows-benchmarks.sh:9`
(`ci.${CI_PROJECT_NAME}.dd_api_key-prod`).

## Implementation details

- `run-benchmarks.ps1` now guards the agentless toggle on `DD_API_KEY`
being set, so local/dev invocations without a key continue to behave as
before (warn and skip agentless) rather than attempting — and failing —
to hit intake.
- No changes to `Build.cs`, tracer code, or the benchmark projects. This
is purely a CI config fix.

## Test coverage

- Verified the execution order in `bp-runner.windows.yml`: the env vars
are set in `run-benchmarks.ps1` after
`BuildTracerHome`/`BuildBenchmarks` and before the BDN CLI is invoked —
i.e., inherited by every BDN child process.

## Other details

Author: faydef

.gitlab/benchmarks/microbenchmarks.yml

@@ -106,7 +106,7 @@ run-benchmarks:
     - mkdir -p artifacts
     - export GITHUB_TOKEN=$(cat /tmp/github-token)
     # Fetch DD_API_KEY from SSM (in GitLab runner's AWS account) and export for bp-infra
-    - export DD_API_KEY=$(aws ssm get-parameter --name "ci.dd-trace-dotnet.dd_api_key" --with-decryption --query "Parameter.Value" --output text --region "${AWS_REGION}")
+    - export DD_API_KEY=$(aws ssm get-parameter --name "ci.dd-trace-dotnet.dd_api_key-prod" --with-decryption --query "Parameter.Value" --output text --region "${AWS_REGION}")
     - CLEANUP_ARG=$([[ "$CLEANUP" == "false" ]] && echo "--no-cleanup" || echo "")
     # Run benchmarks on ephemeral instance
     - |

.gitlab/benchmarks/microbenchmarks/scripts/run-benchmarks.ps1

@@ -114,6 +114,19 @@ $env:DD_ENV = "CI"
 $env:DD_DOTNET_TRACER_HOME = $monitoringHome
 $env:DD_TRACER_HOME = $monitoringHome
 
+# CI Visibility ships benchmark results to Datadog via the in-process tracer.
+# The ephemeral benchmarking VM does not run a Datadog Agent, so route directly
+# to intake via agentless mode. DD_API_KEY is forwarded from the GitLab job.
+if ($env:DD_API_KEY) {
+    $env:DD_CIVISIBILITY_AGENTLESS_ENABLED = "1"
+    if (-not $env:DD_SITE) {
+        $env:DD_SITE = "datadoghq.com"
+    }
+    Write-Output "CI Visibility agentless mode enabled (site: $env:DD_SITE)"
+} else {
+    Write-Warning "DD_API_KEY not set; CI Visibility data will not be sent to Datadog"
+}
+
 # Build BenchmarkDotNet arguments
 $arguments = @("-r") + $runtimes + @(
     "-m",