docs(tutorial): fix stale registry prose in instrumenting HTTP server

[santhiprakash]

Fixes #2815

Problem

Three statements in docs/tutorials/instrumenting_http_server_in_go.md still describe the pre-292bc326 version of the sample, which used the default registry. The sample code in the same file now builds a custom registry (prometheus.NewRegistry() + promauto.With(reg) + promhttp.HandlerFor(reg, …)), so the prose contradicts the code it explains.

Fix

• "The prometheus.MustRegister function registers the pingCounter with the default registry." → promauto.With(reg).NewCounter() creates the counter and registers it with reg.
• "promhttp.Handler() … exposes the metrics registered in the default registry." → promhttp.HandlerFor(reg, promhttp.HandlerOpts{}) exposes the metrics registered in reg.
• "The default registry comes with a collector for Go runtime metrics…" → the custom registry does not include Go runtime metrics by default; to collect them, register collectors.NewGoCollector() with the registry (the non-deprecated API, per @jan--f's review on docs: fix stale explanation in instrumenting HTTP server tutorial #3074).

Verification

• Ran the tutorial's exact sample code against client_golang v1.24.1: after three /ping requests, GET /metrics returns only ping_request_count 3 — no go_*, promhttp_*, or process_* series, confirming prometheus.NewRegistry() starts empty.
• make check (mdox fmt + link validation) passes.
• The diff is limited to the three sentences; ping_metric.png and the Help string it renders are left unchanged so the text and image stay in sync.

Notes / Risks

• Docs-only change.
• The go_*/promhttp_* series visible in ping_metric.png predate the custom-registry switch; happy to regenerate the screenshot in a follow-up if you'd like it to match the current sample.

Assistance

AI-assisted. I wrote and verified this change.