feat: Content analytics — Prometheus counter + OTel span attributes

[khaines]

Summary

BlogFlow's HTTP metrics use low-cardinality route patterns (GET /posts/{slug}) for system health, but cannot answer "which posts are popular?" or "what content gets the most traffic?".

Proposed Solution

Two complementary, cloud-agnostic signals:

A. Prometheus counter

blogflow_content_views_total{type, slug} incremented on every successful content serve. Enables PromQL queries like "top 10 posts this week".

Labels:

• type: post, page, tag, list, home
• slug: actual slug/tag for posts/pages/tags; empty for list/home

Cardinality: ~1 series per content item (500 posts → 500 series — well within Prometheus comfort zone).

C. OTel span attributes

Added to the existing otelhttp request span:

Attribute	Type	Example
content.type	string	post
content.slug	string	hello-world
content.title	string	Hello World
content.tags	string[]	["go","blog"]

Works with any OTel-compatible backend (Jaeger, Tempo, Zipkin, Grafana Cloud, etc.).

Implementation

• New internal/server/handlers/content_metrics.go with counter + RecordContentView() helper
• Wire to all 6 content handlers (PostHandler, PageHandler, TagHandler, PostsListHandler, ListHandler, HomeHandler)
• Zero config — lights up automatically when content is served
• Unit tests for counter increments and span attribute population

Acceptance Criteria

• blogflow_content_views_total counter increments on each content serve
• OTel span attributes populated for all content requests
• Only recorded on successful content lookup (no 404 pollution)
• Unit tests pass
• go test ./... passes