Segment membership inspection metrics and logs

[khvn26]

Scope

Implement the metrics and structured logs named in the RFC's Observability section:

• Counters: flagsmith_segment_membership_atom_evaluations_total, flagsmith_segment_membership_bitmap_ops_total, flagsmith_segment_membership_backfill_identities_total.
• Histograms: flagsmith_segment_membership_read_latency_seconds, flagsmith_segment_membership_write_latency_seconds, flagsmith_segment_membership_write_to_visible_seconds, flagsmith_segment_membership_backfill_duration_seconds.
• Gauges: flagsmith_segment_membership_atoms_per_env, flagsmith_segment_membership_bitmap_bytes_per_env.
• Structlog events: segment_membership.atom.created, segment_membership.atom.backfilled, segment_membership.query.served, segment_membership.backfill.failed.

Acceptance criteria

• Metrics show up in /metrics after the index has handled at least one read / write / backfill.
• Histogram bucket boundaries cover the SLO targets in flagsmith/pulumi#208.
• Each metric carries a description following the api/README.md "Code guidelines: metrics" naming.