Ingesting logs to Loki using OpenTelemetry Collector

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Here are some example configs to change the default mapping of OTLP to Loki format:

Example 1

yaml

limits_config:
  otlp_config:
    resource_attributes:
      attributes_config:
        - action: index_label
          attributes:
            - service.group

With the example config, here is how various kinds of Attributes would be stored:

Store all 17 Resource Attributes mentioned earlier and service.group Resource Attribute as index labels.
Store remaining Resource Attributes as Structured Metadata.
Store all the Scope and Log Attributes as Structured Metadata.

Example 2

yaml

limits_config:
  otlp_config:
    resource_attributes:
      ignore_defaults: true
      attributes_config:
        - action: index_label
          regex: service.group

With the example config, here is how various kinds of Attributes would be stored:

Only store service.group Resource Attribute as index labels.
Store remaining Resource Attributes as Structured Metadata.
Store all the Scope and Log Attributes as Structured Metadata.

Example 3

yaml

limits_config:
  otlp_config:
    resource_attributes:
      attributes_config:
        - action: index_label
          regex: service.group
    scope_attributes:
      - action: drop
        attributes:
          - method.name
    log_attributes:
      - action: structured_metadata
        attributes:
          - user.id
      - action: drop
        regex: .*

With the example config, here is how various kinds of Attributes would be stored:

Store all 17 Resource Attributes mentioned earlier and service.group Resource Attribute as index labels.
Store remaining Resource Attributes as Structured Metadata.
Drop Scope Attribute named method.name and store all other Scope Attributes as Structured Metadata.
Store Log Attribute named user.id as Structured Metadata and drop all other Log Attributes.

Handle large attributes such as stack traces

By default, Loki stores OpenTelemetry log attributes as structured metadata. Two per-line limits apply to them:

max_structured_metadata_size: the total size of structured metadata per log line, 64KB by default.
max_structured_metadata_entries_count: the number of structured metadata entries per log line, 128 by default.

A large log attribute — for example exception.stacktrace, which some OpenTelemetry SDK log integrations attach when a log records an exception — can push a log line over these limits. The distributor rejects such entries with an HTTP 400 (non-retryable) response and counts them in the loki_discarded_samples_total metric with reason structured_metadata_too_large or structured_metadata_too_many; other valid entries in the same push request may still be ingested. For the exact error messages, refer to Troubleshoot ingestion errors.

If your stack traces are being rejected, use one of the following mitigations.

Move the stack trace into the log body

Use the OpenTelemetry Collector transform processor to append the stack trace to the log body and delete the attribute:

yaml

processors:
  transform/stacktrace:
    log_statements:
      - context: log
        statements:
          - set(body, Concat([body, attributes["exception.stacktrace"]], "\n")) where attributes["exception.stacktrace"] != nil
          - delete_key(attributes, "exception.stacktrace") where attributes["exception.stacktrace"] != nil
service:
  pipelines:
    logs:
      processors: [transform/stacktrace] # append to your existing processors list

The set statement must come before delete_key. The stack trace then counts against the log line size limit instead: max_line_size, 256KB by default, with max_line_size_truncate available to truncate rather than reject oversized lines. If you collect logs with Grafana Alloy, the otelcol.processor.transform component applies the same statements.

Drop the attribute or adjust the limits

If you don't need stack traces in Loki, drop the attribute at ingest with a log_attributes rule using action: drop under limits_config.otlp_config (Example 3 above shows the pattern). Rules are applied first-match-wins, so place the specific attribute before any catch-all regex.
On self-managed Loki, you can raise max_structured_metadata_size or max_structured_metadata_entries_count for a tenant through runtime overrides.
On Grafana Cloud, contact support to discuss limit adjustments.