Summary

• Add HostFunc option following the same pattern as HandlerNameFunc and RequestPathFunc
• Allow users to override the host value used in Prometheus metric labels (both counter and histogram) to reduce cardinality when the Host header varies (e.g. dynamic subdomains)
• When not set, defaults to c.Request.Host (preserving current behavior)

Motivation

When services handle requests with dynamic subdomains (e.g. 12345.example.com), the host label in Prometheus metrics causes high cardinality. Currently there is no way to customize the host label without mutating c.Request.Host via external middleware before instrumentation.

This adds a HostFunc option that lets users normalize the host at the metric level:

p := ginprom.New(
    ginprom.HostFunc(func(c *gin.Context) string {
        return stripDynamicSubdomain(c.Request.Host)
    }),
)

Test plan

• Added TestHostFunc following the same pattern as TestHandlerNameFunc
• All existing tests pass

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