exporter: rename scrapeall instance label

[SAY-5]

Fixes #1143.

currently adds an label to each target's metrics, which Prometheus rewrites to its reserved label. That hides the MongoDB target identity and makes the label unusable in PromQL.

This change renames the label to , updates the README example, and adds a small unit test for the label helper.

Validation:

• ok github.com/percona/mongodb_exporter/exporter (cached)
• ok github.com/percona/mongodb_exporter (cached)
  ok github.com/percona/mongodb_exporter/exporter (cached) [no tests to run]
  ok github.com/percona/mongodb_exporter/exporter/dsn_fix (cached) [no tests to run]
  ? github.com/percona/mongodb_exporter/internal/proto [no test files]
  ok github.com/percona/mongodb_exporter/internal/tu (cached) [no tests to run]
  ? github.com/percona/mongodb_exporter/internal/util [no test files]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.33%. Comparing base (dc46ed5) to head (156eabc).
⚠️ Report is 220 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (dc46ed5) and HEAD (156eabc). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (dc46ed5)	HEAD (156eabc)
agent	10	6

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1286      +/-   ##
==========================================
- Coverage   70.88%   65.33%   -5.56%     
==========================================
  Files          28       29       +1     
  Lines        3569     2481    -1088     
==========================================
- Hits         2530     1621     -909     
+ Misses        904      715     -189     
- Partials      135      145      +10     

Flag	Coverage Δ
agent	65.33% <100.00%> (-5.56%)	⬇️

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