Add OTel trace group processor to doc website.

_data-prepper/pipelines/configuration/processors/otel-trace-group.md

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+---
+layout: default
+title: otel_trace_group processor
+parent: Processors
+grand_parent: Pipelines
+nav_order: 45
+---
+
+# otel_trace_group processor
+
+The `otel_trace_group` processor completes missing trace-group-related fields in the collection of [span](https://github.com/opensearch-project/data-prepper/blob/834f28fdf1df6d42a6666e91e6407474b88e7ec6/data-prepper-api/src/main/java/org/opensearch/dataprepper/model/trace/Span.java) records by looking up the OpenSearch backend. The `otel_trace_group` processor identifies the missing trace group information for a `spanId` by looking up the relevant fields in its root `span` stored in OpenSearch.
+
+## OpenSearch
+
+When you connect to an OpenSearch cluster using your username and password, use the following `pipeline.yaml` file example to configure the `otel_trace_group` processor:
+
+``` YAML
+pipeline:
+  ...
+  processor:
+    - otel_trace_group:
+        hosts: ["https://localhost:9200"]
+        cert: path/to/cert
+        username: YOUR_USERNAME_HERE
+        password: YOUR_PASSWORD_HERE
+```
+
+See [OpenSearch security]({{site.url}}{{site.baseurl}}/data-prepper/pipelines/configuration/sinks/opensearch/#opensearch-cluster-security) for a more detailed explanation of which OpenSearch credentials and permissions are required and how to configure those credentials for the OTel trace group processor.
+
+### Amazon OpenSearch Service
+
+When you use [Amazon OpenSearch Service]({{site.url}}{{site.baseurl}}/data-prepper/pipelines/configuration/sinks/opensearch/#amazon-opensearch-service-domain-security), use the following `pipeline.yaml` file example to configure the `otel_trace_group` processor:
+
+``` YAML
+pipeline:
+  ...
+  processor:
+    - otel_trace_group:
+        hosts: ["https://your-amazon-opensearch-service-endpoint"]
+        aws_sigv4: true
+        cert: path/to/cert
+        insecure: false
+```
+
+## Configuration
+
+You can configure the `otel_trace_group` processor with the following options.
+
+| Name | Description | Default value |
+| -----| ----| -----------|
+| `hosts`| A list of IP addresses of OpenSearch nodes. Required. | No default value. | 
+| `cert` | A certificate authority (CA) certificate that is PEM encoded. Accepts both .pem or .crt. This enables the client to trust the CA that has signed the certificate that OpenSearch is using. | `null` |
+| `aws_sigv4` | A Boolean flag used to sign the HTTP request with AWS credentials. Only applies to Amazon OpenSearch Service. See [OpenSearch security](https://github.com/opensearch-project/data-prepper/blob/129524227779ee35a327c27c3098d550d7256df1/data-prepper-plugins/opensearch/security.md) for details. | `false`. |
+| `aws_region` | A string that represents the AWS Region of the Amazon OpenSearch Service domain, for example, `us-west-2`. Only applies to Amazon OpenSearch Service. | `us-east-1` |
+| `aws_sts_role_arn`| An AWS Identity and Access Management (IAM) role that the sink plugin assumes to sign the request to Amazon OpenSearch Service. If not provided, the plugin uses the [default credentials](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/auth/credentials/DefaultCredentialsProvider.html). | `null` |
+| `aws_sts_header_overrides` | A map of header overrides that the IAM role assumes for the sink plugin. | `null` |
+| `insecure` | A Boolean flag used to turn off SSL certificate verification. If set to `true`, CA certificate verification is turned off and insecure HTTP requests are sent. | `false` |
+| `username` | A string that contains the username and is used in the [internal users](https://opensearch.org/docs/latest/security/access-control/users-roles/) `YAML` configuration file of your OpenSearch cluster. | `null` |
+| `password` | A string that contains the password and is used in the [internal users](https://opensearch.org/docs/latest/security/access-control/users-roles/) `YAML` configuration file of your OpenSearch cluster. | `null` |
+
+## Configuration option examples
+
+You can define the configuration option values in the `aws_sts_header_overrides` option. See the following example:
+
+```
+aws_sts_header_overrides:
+  x-my-custom-header-1: my-custom-value-1
+  x-my-custom-header-2: my-custom-value-2
+```
+
+## Metrics
+
+The following table describes custom metrics specific to the `otel_trace_group` processor.
+
+| Metric name | Type | Description |
+| ------------- | ---- | ----------- |
+| `recordsInMissingTraceGroup` | Counter | The number of ingress records missing trace group fields. |
+| `recordsOutFixedTraceGroup` | Counter | The number of egress records with successfully completed trace group fields. |
+| `recordsOutMissingTraceGroup` | Counter | The number of egress records missing trace group fields. |