line: parse DogStatsD container ID field on lines without tags

[Vasu-Madaan]

Problem

The DogStatsD v1.2 container ID field (|c:<id>) can appear on a line that has no |# tags (for example a DogStatsD client with origin/container detection enabled but no constant tags configured). A counter then looks like:

my_counter:1|c|c:0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef

LineToEvents only treats a line as DogStatsD (and skips the legacy multi-metric : split) when it contains |#. Without tags, the line falls through to strings.Split(elements[1], ":"), which splits on every colon and shreds the container ID field:

"1|c|c:0123...ef"  ->  ["1|c|c", "0123...ef"]

The first fragment 1|c|c parses value=1, type=c, and a trailing bare c that fails the container-ID check (malformed_container_id); the second fragment has too few parts (malformed_component). The sample is dropped and the metric is never exported, even though the same line with tags parses correctly:

my_counter:1|c|#env:prod|c:0123...ef   ->  ok, container_id label set

Fix

Detect the container ID field (always the final |-delimited field) and, like |# tags, skip the legacy : split for such lines. A valid legacy multi-metric line always ends in a bare stat type (c, g, ms, ...), never c:<id>, so legacy parsing is unaffected.

Tests

Adds LineToEvents cases for counter/gauge/timer/complex-value container-ID lines without tags. gofmt, go vet, and go test ./... pass.

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Note: Claude was used to help investigate and prepare this change.