feat: OM2 writer outputs names as provided, no suffix appending

[zeitlinger]

Summary

The OM2 writer now uses expositionBaseName directly instead of
appending _total (counters) or unit suffixes. The _info suffix
is enforced per the OM2 spec (MUST). Part of #1942.

Key table

User provides	OM1	OM2
Counter("events")	events_total	events
Counter("events_total")	events_total	events_total
Counter("req").unit(BYTES)	req_bytes_total	req_bytes
Counter("req_bytes").unit(BYTES)	req_bytes_total	req_bytes
Gauge("events_total")	events_total	events_total
Info("target")	target_info	target_info

PR stack

1. Core model + OM1/protobuf writers (feat: move suffix handling to scrape time #1955)
2. OTel preserve_names (independent)
3. This PR — OM2 writer no-suffix

Test plan

• mise run compile passes
• OM2-specific tests verify no _total/unit suffix appending
• _info suffix enforcement tested

Part of #1912.

[jaydeluca]

how should info metrics work?

if we add a test:

  @Test
  void testInfoMetricEnforcesInfoSuffix() throws IOException {
    // User provides "jvm" → OM2 outputs "jvm_info"
    MetricSnapshots snapshots = MetricSnapshots.of(
      InfoSnapshot.builder().name("jvm").help("JVM info")
        .dataPoint(InfoSnapshot.InfoDataPointSnapshot.builder().build()).build());

    String om2Output = writeWithOM2(snapshots);

    // is this right?
    assertThat(om2Output).contains("# TYPE jvm info");
    assertThat(om2Output).contains("jvm_info 1");
  }

the result is:

# TYPE jvm info
# HELP jvm JVM info
jvm_info 1
# EOF

The writeInfo method enforces _info suffix, then removes it for the TYPE line, and then writes the time series with the full _info suffix

Do we require the TYPE line to use the base name or the full name?

[zeitlinger]

good point - om2 should never strip - because metric names are never mutated.

Fixed and added a new test case.