documenting logs bridge api, adding memory exporter (#959)

* implement bridge api

* adding logs memory exporter

* clarify bridge api not for developers

* adding logger provider builder

* review feedback

* removing Bridge and documenting what a bridge is

Author: brettmc

Logs/README.md

@@ -0,0 +1,19 @@
+# Logs API
+
+This `Logger` API is not designed to be used by application developers, but rather by library developers for the purpose
+of integrating existing logging libraries with OpenTelemetry.
+
+## Logging from 3rd party loggers
+
+3rd party loggers should log to OpenTelemetry in accordance with the
+[logs bridge API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/bridge-api.md)
+specification.
+
+This means that a "log appender" in the 3rd party logging library (sometimes known as a "handler") should:
+- accept an `OpenTelemetry\API\Logs\LoggerProviderInterface`, or obtain a globally registered one from `OpenTelemetry\API\Instrumentation\Globals`
+- obtain a `Logger` from the logger provider (optionally adding any resources that should be associated with logs emitted)
+- convert logs from its own log format into OpenTelemetry's `LogRecord` format
+- send the logs to OpenTelemetry via `Logger::logRecord()`
+
+See [monolog-otel-integration](examples/logs/features/monolog-otel-integration.md) for an example.
+