Define file configuration file format and env var substitution (#3744)

Part of incorporating [OTEP
#225](open-telemetry/oteps#225) into the
specification.

Followup to #3437.

The adds the YAML file format, and leaves an open TODO for the JSON
format discussed in the original OTEP. It also define environment
variable substitution. IMO its necessary to define environment variable
substitution at the same time as file format because they interact in
ways that aren't immediately obvious and need to be prototyped.

The opentelemetry-java implementation of file configuration already
accepts YAML encoding and performs environment variable substitution as
described in this PR. It does not support JSON, and I don't think we
should unless there is good reason. I omitted the JSON file format
because I don't know of any prototypes that permit it. We should add
JSON once a language implements it and explores how environment variable
substitution works with JSON.

specification/configuration/file-configuration.md

@@ -12,6 +12,8 @@ linkTitle: File
 - [Configuration Model](#configuration-model)
   * [Stability Definition](#stability-definition)
 - [Configuration file](#configuration-file)
+  * [YAML file format](#yaml-file-format)
+  * [Environment variable substitution](#environment-variable-substitution)
 - [SDK Configuration](#sdk-configuration)
   * [In-Memory Configuration Model](#in-memory-configuration-model)
   * [Operations](#operations)
@@ -46,12 +48,86 @@ TODO: define stability guarantees and backwards compatibility
 
 ## Configuration file
 
-A configuration file is a file representation of
+A configuration file is a serialized file-based representation of
 the [Configuration Model](#configuration-model).
 
-TODO: define acceptable file formats
+Configuration files SHOULD use one the following serialization formats:
 
-TODO: define environment variable substitution
+### YAML file format
+
+[YAML](https://yaml.org/spec/1.2.2/) configuration files SHOULD follow YAML spec
+revision >= 1.2.
+
+YAML configuration files MUST use file extensions `.yaml` or `.yml`.
+
+TODO: decide if JSON file format is required
+
+### Environment variable substitution
+
+Configuration files support environment variables substitution for references
+which match the following regular expression:
+
+```regexp
+\$\{(?<ENV_NAME>[a-zA-Z_][a-zA-Z0-9_]*)}
+```
+
+The `ENV_NAME` MUST start with an alphabetic or `_` character, and is followed
+by 0 or more alphanumeric or `_` characters.
+
+For example, `${API_KEY}` is valid, while `${1API_KEY}` and `${API_$KEY}` are
+invalid.
+
+Environment variable substitution MUST only apply to scalar values. Mapping keys
+are not candidates for substitution.
+
+If a referenced environment variable is not defined, it MUST be replaced with an
+empty value.
+
+Node types MUST be interpreted after environment variable substitution takes
+place. This ensures the environment string representation of boolean, integer,
+or floating point fields can be properly converted to expected types.
+
+It MUST NOT be possible to inject YAML structures by environment variables. For
+example, references to `INVALID_MAP_VALUE` environment variable below.
+
+For example, consider the following environment variables,
+and [YAML](#yaml-file-format) configuration file:
+
+```shell
+export STRING_VALUE="value"
+export BOOl_VALUE="true"
+export INT_VALUE="1"
+export FLOAT_VALUE="1.1"
+export INVALID_MAP_VALUE"value\nkey:value" # An invalid attempt to inject a map key into the YAML
+```
+
+```yaml
+string_key: ${STRING_VALUE}                           # Valid reference to STRING_VALUE
+other_string_key: "${STRING_VALUE}"                   # Valid reference to STRING_VALUE inside double quotes
+another_string_key: "${BOOl_VALUE}"                   # Valid reference to BOOl_VALUE inside double quotes
+yet_another_string_key: ${INVALID_MAP_VALUE}          # Valid reference to INVALID_MAP_VALUE, but YAML structure from INVALID_MAP_VALUE MUST NOT be injected
+bool_key: ${BOOl_VALUE}                               # Valid reference to BOOl_VALUE
+int_key: ${INT_VALUE}                                 # Valid reference to INT_VALUE
+float_key: ${FLOAT_VALUE}                             # Valid reference to FLOAT_VALUE
+combo_string_key: foo ${STRING_VALUE} ${FLOAT_VALUE}  # Valid reference to STRING_VALUE and FLOAT_VALUE
+undefined_key: ${UNDEFINED_KEY}                       # Invalid reference, UNDEFINED_KEY is not defined and is replaced with ""
+${STRING_VALUE}: value                                # Invalid reference, substitution is not valid in mapping keys and reference is ignored
+```
+
+Environment variable substitution results in the following YAML:
+
+```yaml
+string_key: value                           # Interpreted as type string, tag URI tag:yaml.org,2002:str
+other_string_key: "value"                   # Interpreted as type string, tag URI tag:yaml.org,2002:str
+another_string_key: "true"                  # Interpreted as type string, tag URI tag:yaml.org,2002:str
+yet_another_string_key: "value\nkey:value"  # Interpreted as type string, tag URI tag:yaml.org,2002:str
+bool_key: true                              # Interpreted as type bool, tag URI tag:yaml.org,2002:bool
+int_key: 1                                  # Interpreted as type int, tag URI tag:yaml.org,2002:int
+float_key: 1.1                              # Interpreted as type float, tag URI tag:yaml.org,2002:float
+combo_string_key: foo value 1.1             # Interpreted as type string, tag URI tag:yaml.org,2002:str
+undefined_key:                              # Interpreted as type null, tag URI tag:yaml.org,2002:null
+${STRING_VALUE}: value                      # Interpreted as type string, tag URI tag:yaml.org,2002:str
+```
 
 ## SDK Configuration
 
@@ -81,10 +157,19 @@ with OpAmp
 
 Parse and validate a [configuration file](#configuration-file).
 
+Parse MUST perform [environment variable substitution](#environment-variable-substitution).
+
+Parse MUST interpret null as equivalent to unset.
+
 **Parameters:**
 
 * `file`: The [configuration file](#configuration-file) to parse. This MAY be a
   file path, or language specific file data structure, or a stream of a file's content.
+* `file_format`: The file format of the `file` (e.g. [yaml](#yaml-file-format)).
+  Implementations MAY accept a `file_format` parameter, or infer it from
+  the `file` extension, or include file format specific overloads of `parse`,
+  e.g. `parseYaml(file)`. If `parse` accepts `file_format`, the API SHOULD be
+  structured so a user is obligated to provide it.
 
 **Returns:** [configuration model](#in-memory-configuration-model)
 
@@ -94,12 +179,19 @@ This SHOULD return an error if:
 * The parsed `file` content does not conform to
   the [configuration model](#configuration-model) schema.
 
-TODO: define behavior if some portion of configuration model is not supported
-
 #### Create
 
 Interpret [configuration model](#in-memory-configuration-model) and return SDK components.
 
+If a field is null or unset and a default value is defined, Create MUST ensure
+the SDK component is configured with the default value. If a field is null or
+unset and no default value is defined, Create SHOULD return an error. For
+example, if configuring
+the [span batching processor](../trace/sdk.md#batching-processor) and
+the `scheduleDelayMillis` field is null or unset, the component is configured
+with the default value of `5000`. However, if the `exporter` field is null or
+unset, Create fails fast since there is no default value for `exporter`.
+
 **Parameters:**
 
 * `configuration` - The configuration model.
@@ -115,7 +207,10 @@ The multiple responses MAY be returned using a tuple, or some other data
 structure encapsulating the components.
 
 This SHOULD return an error if it encounters an error in `configuration` (i.e.
-fail fast).
+fail fast) in accordance with
+initialization [error handling principles](../error-handling.md#basic-error-handling-principles).
+
+TODO: define behavior if some portion of configuration model is not supported
 
 ## References