docs: fix stale explanation in instrumenting HTTP server tutorial

[santhiprakash]

Problem

The explanation paragraph in the Go instrumenting tutorial still references
MustRegister, promhttp.Handler(), and the "default registry" — none of
which appear in the code. The code was changed to use prometheus.NewRegistry()
(custom registry) in Nov 2022 (commit 292bc32), but the surrounding prose was
never updated. Subsequent grammar fixes (PR #2824, merged) cleaned up style but
left the stale API references intact.

Additionally:

• The Help text says "No of request" in the full sample code but "No of
  requests" in the incremental code block above.
• The text claims Go runtime metrics appear because "the default registry comes
  with a collector," but the code uses a custom registry that does not include
  them.

Partially addresses #2815 (PR #2824 fixed grammar but these correctness issues
remained).

Fix

• Update the explanation paragraph to reference promauto.With(reg),
  promhttp.HandlerFor(reg, ...), and "custom registry" — matching the
  actual code.
• Fix the Help text inconsistency: "No of request" → "No of requests" in
  the full sample code.
• Replace the misleading Go runtime metrics claim with an accurate note that
  the custom registry does not include them by default and how to add them.

Verification

• Diff read against upstream/main (12e2edf).
• Verified all changed prose references functions and patterns that exist in
  the code blocks on the same page.
• Cross-checked the git history: commit 292bc32 (Nov 2022) introduced the
  custom registry code but left the old prose untouched.

Notes

• The screenshot showing Go runtime metrics (go_threads, go_goroutines)
  predates the custom-registry change and is now slightly misleading. An
  updated screenshot would be ideal but is out of scope for this text-only fix.