Disable agent.* metrics

[axw]

Disable the agent.* metrics, as they were ill-defined.
We will reinstate these metrics, or some alternatives,
at a later date after design and discussion with other
agent developers.

Closes #396

[codecov-io]

Codecov Report

Merging #407 into master will increase coverage by 3.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
+ Coverage   80.63%   83.95%   +3.31%     
==========================================
  Files         102      102              
  Lines        6136     6126      -10     
==========================================
+ Hits         4948     5143     +195     
+ Misses        914      913       -1     
+ Partials      274       70     -204

Impacted Files	Coverage Δ
builtin_metrics.go	91.56% <ø> (+6.45%)	⬆️
modelwriter.go	94.95% <0%> (+0.84%)	⬆️
internal/radix/radix.go	39.92% <0%> (+0.92%)	⬆️
utils.go	80.68% <0%> (+1.13%)	⬆️
internal/sqlscanner/scanner.go	96.82% <0%> (+1.58%)	⬆️
metrics.go	91.66% <0%> (+1.66%)	⬆️
error.go	77.45% <0%> (+1.73%)	⬆️
context.go	94.05% <0%> (+1.98%)	⬆️
internal/apmhttputil/url.go	97.87% <0%> (+2.12%)	⬆️
module/apmsql/stmt.go	63.82% <0%> (+2.12%)	⬆️
... and 32 more

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