webmachine_util: monotonic-time-aware now_diff helpers

[samwar]

Summary

Finishes migrating webmachine_util:now_diff_milliseconds/2 off os:timestamp() tuples and onto the erlang:monotonic_time() native-integer inputs the wm_log_data record has been carrying for a while. Adds a matching now_diff_microseconds/2 so downstream access-log handlers don't need to duplicate the conversion.

Motivation

The wm_log_data record's start_time, end_time, and finish_time fields were migrated from os:timestamp() tuples to erlang:monotonic_time() integers in native units. Internal callers (webmachine_access_log_handler, webmachine_perf_log_handler) followed suit and moved to the new webmachine_log:{request,post}_processing_time/1 helpers.

What got left behind was webmachine_util:now_diff_milliseconds/2 — still exported public API, still tuple-only:

now_diff_milliseconds({M,S,U}, {M,S1,U1}) ->
    ((S-S1) * 1000) + ((U-U1) div 1000);
now_diff_milliseconds({M,S,U}, {M1,S1,U1}) ->
    ((M-M1)*1000000+(S-S1))*1000 + ((U-U1) div 1000).

Any downstream consumer with its own gen_event handler that reads start_time/end_time off wm_log_data and diffs them via this function will hit no function clause matching now_diff_milliseconds(<neg-integer>, <neg-integer>) on every request.

Changes

• now_diff_milliseconds/2 — replace tuple clauses with an integer-input path that delegates to erlang:convert_time_unit(Diff, native, millisecond). Undefined "later" time is treated as erlang:monotonic_time() — same convention webmachine_log:performance_time_diff/2 uses.
• now_diff_microseconds/2 — new, matching shape. Matches the older timer:now_diff/2 return unit for consumers that log response times at microsecond granularity.
• diff_native/2 — private helper factored out so both public functions share the same undefined-argument handling.
• Eunit tests updated to construct start/end times as native-unit integers and cover both the millisecond and microsecond paths, plus the undefined branch on each.

Compatibility

Tuple callers will no longer match. In practice, no such callers should exist:

• The record migration to native-unit integers means anyone reading start_time/end_time off the record has been passing integers to now_diff_milliseconds/2 since that migration and getting silent runtime crashes.
• Internal webmachine callers all moved to webmachine_log:{request,post}_processing_time/1 in the same wave.

This PR turns silent breakage into an explicit signal to update.

Tests

Six eunit test functions cover the helpers. Two are new and address the specific coverage gap that let the tuple-based clauses stay green after the record migrated to native integers.

Existing coverage:

• now_diff_milliseconds_test — synthetic integers, exact ms diff.
• now_diff_milliseconds_undefined_test — the undefined-later-time branch.
• now_diff_microseconds_test — synthetic integers, exact μs diff.
• now_diff_microseconds_undefined_test — undefined branch on the microsecond path.

New coverage:

• now_diff_wm_log_data_shape_test — constructs a real #wm_log_data{} with erlang:monotonic_time() values and diffs the fields via the new helpers. Requires the record definition (webmachine_logger.hrl) at test-compile time, which is what makes the contract binding: if the record's field type ever diverges from what the helpers accept, this test breaks at compile time (record construction) or runtime (function clause), whichever comes first. This is the check that would have caught the situation where the record moved to integers but now_diff_milliseconds/2 still only pattern-matched on tuples.
• now_diff_milliseconds_realtime_test — timer:sleep(15) between two erlang:monotonic_time/0 calls, then asserts the diff is ≥ 15 ms and < 1000 ms. Catches unit-conversion regressions where a mis-typed target unit (e.g. nanosecond, second) would silently return a nonsensical number.

Verification

• erlc -DTEST -I include src/webmachine_util.erl: clean compile.
• All six new/updated now_diff_* tests pass.
• Other pre-existing failures in the module's eunit suite (media_type_extra_whitespace_test, format_content_type_test, parse_range_test) are unrelated — they require mochiweb_util/mochiweb_http which weren't on the standalone erl -pa sandbox path.