feat(apm): add referrer hostname as span tag

[cgcote]

Parses referrer hostname from http header when presents and adds it to span tags as http.referrer_hostname. This will be used to identify frontend clients without leaking the PII which can be contained in the entire referrer.

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/referrer-hostname-8aa6dc6edb9d53b1.yaml              @DataDog/apm-python
ddtrace/contrib/internal/trace_utils.py                                 @DataDog/apm-core-python @DataDog/apm-idm-python
ddtrace/ext/http.py                                                     @DataDog/apm-core-python @DataDog/apm-idm-python
tests/tracer/test_trace_utils.py                                        @DataDog/apm-sdk-api-python

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 232 ± 4 ms.

The average import time from base is: 233 ± 4 ms.

The import time difference between this PR and base is: -0.5 ± 0.2 ms.

Import time breakdown

The following import paths have grown:

ddtrace.auto 1.186 ms (0.51%)

ddtrace.bootstrap.sitecustomize 1.098 ms (0.47%)

ddtrace.bootstrap.preload 1.098 ms (0.47%)

ddtrace.internal.products 1.098 ms (0.47%)

importlib.metadata 0.936 ms (0.40%)

importlib.abc 0.752 ms (0.32%)

importlib.resources 0.752 ms (0.32%)

importlib.resources._legacy 0.752 ms (0.32%)

csv 0.114 ms (0.05%)

importlib.metadata._itertools 0.069 ms (0.03%)

ddtrace.internal.remoteconfig._connectors 0.085 ms (0.04%)

ctypes 0.085 ms (0.04%)

_ctypes 0.085 ms (0.04%)

ddtrace.debugging._config 0.077 ms (0.03%)

ddtrace.settings.exception_replay 0.077 ms (0.03%)

ddtrace 0.088 ms (0.04%)

ddtrace.trace 0.088 ms (0.04%)

ddtrace._trace.tracer 0.088 ms (0.04%)

ddtrace.internal.processor.endpoint_call_counter 0.088 ms (0.04%)

The following import paths have shrunk:

ddtrace.auto 2.897 ms (1.25%)

ddtrace.bootstrap.sitecustomize 2.214 ms (0.95%)

ddtrace.bootstrap.preload 2.214 ms (0.95%)

ddtrace.internal.products 2.214 ms (0.95%)

importlib.metadata 0.896 ms (0.39%)

importlib.abc 0.785 ms (0.34%)

importlib.resources 0.063 ms (0.03%)

csv 0.110 ms (0.05%)

_csv 0.110 ms (0.05%)

ddtrace.internal.remoteconfig.client 0.616 ms (0.27%)

ddtrace.internal.remoteconfig._connectors 0.065 ms (0.03%)

ctypes 0.065 ms (0.03%)

ddtrace 0.683 ms (0.29%)

ddtrace.trace 0.080 ms (0.03%)

ddtrace._trace.tracer 0.080 ms (0.03%)

ddtrace.internal.schema.processor 0.080 ms (0.03%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-02 09:34:48

Comparing candidate commit 8ab9a24 in PR branch charles.cote/referrer_host with baseline commit 961a8dc in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 498 metrics, 2 unstable metrics.

[mabdinur]

Left some non blocking suggestions

[labbati]

SGTM!

[labbati]

SGTM