[YSQL] Import function to log the memory contexts of specified backend process.

[sushantrmishra]

Jira Link: DB-3524

Description

Import commit to log memory context information of PostgreSQL usage

PostgreSQL commit -- https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=43620e328617c1f41a2a54c8cee01723064e3ffa

For YugabyteDB we will have the following extra rows in memory context view.

• StorageGatewayMemoryContext : Displays the Memory consumed by pggate layer
• AllocatorCachedMemoryContext: Displays the Memory cached by tcmalloc

# select name, total_bytes, used_bytes from pg_get_backend_memory_contexts() order by total_bytes desc limit 4;
-[ RECORD 1 ]-------------------
name        | CacheMemoryContext
total_bytes | 12853648
used_bytes  | 10271072
-[ RECORD 2 ]-------------------
name        | TopMemoryContext
total_bytes | 389872
used_bytes  | 357392
-[ RECORD 3 ]-------------------
name        | StorageGatewayMemoryContext
total_bytes | 303104
used_bytes  | 303104
-[ RECORD 4 ]-------------------
name        |  AllocatorCachedMemoryContext
total_bytes | 294912
used_bytes  | 294912