api: add new API flb_service_set()

Signed-off-by: Takahiro YAMASHITA <nokute78@gmail.com>

Author: nokute78

development/library_api.md

@@ -6,7 +6,7 @@
 
 [Fluent Bit](http://fluentbit.io) runs as a service, meaning that the API exposed for developers provide interfaces to create and manage a context, specify inputs/outputs, set configuration parameters and set routing paths for the event/records. A typical usage of the library involves:
 
-- Create library instance/context.
+- Create library instance/context and set properties.
 - Enable _input_ plugin(s) instances and set properties.
 - Enable _output_ plugin(s) instances and set properties.
 - Start the library runtime.
@@ -53,6 +53,34 @@ if (!ctx) {
 }
 ```
 
+### Set Service Properties
+
+Using the __flb_service_set()__ function is possible to set context properties.
+
+__Prototype__
+
+```C
+int flb_service_set(flb_ctx_t *ctx, ...);
+```
+
+__Return Value__
+
+On success it returns 0; on error it returns a negative number.
+
+__Usage__
+
+The __flb_service_set()__ allows to set one or more properties in a key/value string mode, e.g:
+
+```C
+int ret;
+
+ret = flb_service_set(ctx, "Flush", "1", NULL);
+
+```
+
+The above example specified the values for the properties __Flush__ , note that the value is always a string (char *) and once there is no more parameters a NULL argument must be added at the end of the list.
+
+
 ### Enable Input Plugin Instance
 
 When built, [Fluent Bit](http://fluentbit.io) library contains a certain number of built-in _input_ plugins. In order to enable an _input_ plugin, the function __flb_input__() is used to create an instance of it.
@@ -97,7 +125,7 @@ On success it returns 0; on error it returns a negative number.
 
 __Usage__
 
-The __flb_input_set()__ allow to set one or more properties in a key/value string mode, e.g:
+The __flb_input_set()__ allows to set one or more properties in a key/value string mode, e.g:
 
 ```C
 int ret;
@@ -156,7 +184,7 @@ On success it returns 0; on error it returns a negative number.
 
 __Usage__
 
-The __flb_output_set()__ allow to set one or more properties in a key/value string mode, e.g:
+The __flb_output_set()__ allows to set one or more properties in a key/value string mode, e.g:
 
 ```C
 int ret;