Adopt GDI specification v1.8.0

[Kielek]

Adopt https://github.com/signalfx/gdi-specification/releases/tag/v1.8.0

[pmcollins]

The following issues were opened to track outstanding gaps:

• CHANGELOG.md missing upstream release note links #697 — CHANGELOG.md missing upstream release note links
• README.md missing GDI specification version badge #698 — README.md missing GDI specification version badge
• README.md missing troubleshooting information #699 — README.md missing troubleshooting information
• Repository documentation gaps (config params, manual instrumentation, log correlation) #700 — Repository documentation gaps (config params, manual instrumentation, log correlation)
• Snapshot profiling: SPLUNK_SNAPSHOT_SELECTION_PROBABILITY has no maximum enforcement #701 — Snapshot profiling: SPLUNK_SNAPSHOT_SELECTION_PROBABILITY has no maximum enforcement
• Integration Context (Cisco/AppD) propagation not implemented #702 — Integration Context (Cisco/AppD) propagation not implemented
• Profiling instrumentation scope version is 0.2.0, spec requires 0.1.0 #704 — Profiling instrumentation scope version is 0.2.0, spec requires 0.1.0

Also noting two upcoming spec changes on the GDI spec main branch (not yet released) that will need to be addressed in a future release:

• Realm ingest URL domain migration from signalfx.com to observability.splunkcloud.com
• Snapshot profiling simplification: removal of splunk.trace.snapshot.volume baggage, simplified trace selection algorithm, max selection rate removed