[graphApi] add non-identity edge annotations with ancestor inheritence

[pbibra]

Summary

• Adds is_annotation: true boolean to edge_metadata_tag_mapping fields in GraphConfig. Annotation fields are excluded from edge identity (not part of metadata or generateKey) so they never split otherwise identical edges.
• Introduces Edge.annotations (Map<String, Set<String>>), accumulating distinct values across observations of the same logical edge. addObservation(Map<String,String>) replaces incrementObservedCount() as
  the single call site, combining count increment and annotation merge.
• Adds GraphConfig.resolveAnnotationsForSpan, which walks the parent chain to find the nearest ancestor where all configured keys fully resolve. Results are memoized so each span's chain is walked at most once. All annotation fields currently inherit from ancestors by default. A future inherit_from_ancestor flag or equivalent on TagConfig could gate this per field.
• Wires annotation resolution into GraphBuilder.traverseAndBuildGraph — only spans that produce edges are walked, keeping it lazy.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.