Clarify OTEL_EXPORTER_OTLP_ENDPOINT envvar for OTLP/HTTP. (#1975)

The current wording of the OTLP endpoint config was confusing, especially the "if not present already" wording. Instead, clarify that we must always append when using the envvar for all signals (it was already clearly specified that the per-signal vars do not get the path appended).

This came up in open-telemetry/opentelemetry-java#3650 and again at open-telemetry/opentelemetry-java#3666 (comment).

Also make it a MUST (not SHOULD) since this kind of thing would be extremely annoying to have differences per-language in. Also, without appending, the variable cannot be used to configure more than one signal which would defeat its sole purpose.

specification/protocol/exporter.md

@@ -10,7 +10,7 @@ The following configuration options MUST be available to configure the OTLP expo
 
 | Configuration Option | Description                                                  | Default           | Env variable                                                 |
 | -------------------- | ------------------------------------------------------------ | ----------------- | ------------------------------------------------------------ |
-| Endpoint (OTLP/HTTP) | Target to which the exporter is going to send spans or metrics. The endpoint MUST be a valid URL with scheme (http or https) and host, and MAY contain a port and path. A scheme of https indicates a secure connection. When using `OTEL_EXPORTER_OTLP_ENDPOINT`, exporters SHOULD follow the collector convention of appending the version and signal to the path (e.g. `v1/traces` or `v1/metrics`), if not present already. The per-signal endpoint configuration options take precedence and can be used to override this behavior. See the [OTLP Specification][otlphttp-req] for more details. | `https://localhost:4318` | `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` |
+| Endpoint (OTLP/HTTP) | Target URL to which the exporter is going to send spans or metrics. The endpoint MUST be a valid URL with scheme (http or https) and host, MAY contain a port, SHOULD contain a path and MUST NOT contain other parts (such as query string or fragment). A scheme of https indicates a secure connection. When using `OTEL_EXPORTER_OTLP_ENDPOINT`, exporters MUST construct per-signal URLs as  [described below](#per-signal-urls). The per-signal endpoint configuration options take precedence and can be used to override this behavior (the URL is used as-is for them, without any modifications). See the [OTLP Specification][otlphttp-req] for more details. | `https://localhost:4318` | `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` |
 | Endpoint (OTLP/gRPC) | Target to which the exporter is going to send spans or metrics. The endpoint SHOULD accept any form allowed by the underlying gRPC client implementation. Additionally, the endpoint MUST accept a URL with a scheme of either `http` or `https`. A scheme of `https` indicates a secure connection and takes precedence over the `insecure` configuration setting. If the gRPC client implementation does not support an endpoint with a scheme of `http` or `https` then the endpoint SHOULD be transformed to the most sensible format for that implementation. | `https://localhost:4317` | `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` |
 | Insecure             | Whether to enable client transport security for the exporter's gRPC connection. This option only applies to OTLP/gRPC - OTLP/HTTP always uses the scheme provided for the `endpoint`. Implementations MAY choose to not implement the `insecure` option if it is not required or supported by the underlying gRPC client implementation. | `false` | `OTEL_EXPORTER_OTLP_INSECURE` `OTEL_EXPORTER_OTLP_SPAN_INSECURE` `OTEL_EXPORTER_OTLP_METRIC_INSECURE` |
 | Certificate File     | The trusted certificate to use when verifying a server's TLS credentials. Should only be used for a secure connection. | n/a               | `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE` |
@@ -28,24 +28,65 @@ Supported values for `OTEL_EXPORTER_OTLP_*COMPRESSION` options:
 - `none` if compression is disabled.
 - `gzip` is the only specified compression method for now.
 
-Example 1
+<a name="per-signal-urls"></a>
+
+### Endpoint URLs for OTLP/HTTP
+
+Based on the environment variables above, the OTLP/HTTP exporter MUST construct URLs
+for each signal as follow:
+
+1. For the per-signal variables (`OTEL_EXPORTER_OTLP_<signal>_ENDPOINT`), the URL
+   MUST be used as-is without any modification. The only exception is that if an
+   URL contains no path part, the root path `/` MUST be used (see [Example 2](#example-2)).
+2. If signals are sent that have no per-signal configuration from the previous point,
+   `OTEL_EXPORTER_OTLP_ENDPOINT` is used as a base URL and the signals are sent
+   to these paths relative to that:
+
+   * Traces: `v1/traces`
+   * Metrics: `v1/metrics`.
+
+   Non-normatively, this could be implemented by ensuring that the base URL ends with
+   a slash and then appending the relative URLs as strings.
+
+#### Example 1
 
 The following configuration sends all signals to the same collector:
 
 ```bash
-export OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4317
+export OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4318
 ```
 
-Example 2
+Traces are sent to `http://collector:4318/v1/traces` and metrics to
+`http://collector:4318/v1/metrics`.
 
-Traces and metrics are sent to different collectors:
+#### Example 2
 
-```bash
-export OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=http://collector:4317
+Traces and metrics are sent to different collectors and paths:
 
+```bash
+export OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=http://collector:4318
 export OTEL_EXPORTER_OTLP_METRICS_ENDPOINT=https://collector.example.com/v1/metrics
 ```
 
+This will send traces directly to the root path `http://collector:4318/`
+(`/v1/traces` is only automatically added when using the non-signal-specific
+environment variable) and metrics
+to `https://collector.example.com/v1/metrics`.
+
+#### Example 3
+
+The following configuration sends all signals except for metrics to the same collector:
+
+```bash
+export OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4318/mycollector/
+export OTEL_EXPORTER_OTLP_METRICS_ENDPOINT=https://collector.example.com/v1/metrics/
+```
+
+Traces are sent to `http://collector:4318/mycollector/v1/traces`
+and metrics to `https://collector.example.com/v1/metrics/`
+(other signals, would they be defined, would be sent to their specific paths
+relative to `http://collector:4318/mycollector/`).
+
 ### Specify Protocol
 
 The `OTEL_EXPORTER_OTLP_PROTOCOL`, `OTEL_EXPORTER_OTLP_TRACES_PROTOCOL`, and `OTEL_EXPORTER_OTLP_METRICS_PROTOCOL` environment variables specify the OTLP transport protocol. Supported values: